Linearly spaced time stepping option

Documentation/ProjectFile/prj/time_loop/processes/process/time_stepping/FixedTimeStepping/t_n_steps.md

+An optional parameter specifying the number of time steps.
+All time steps will be of the same size \f$(t_{\rm end} - t_{\rm initial}) \over n\f$.
+
+For more complex setups see the `timesteps` parameter documentation.

NumLib/TimeStepping/Algorithms/CreateFixedTimeStepping.cpp

@@ -28,6 +28,37 @@ FixedTimeSteppingParameters parseFixedTimeStepping(
     auto const t_initial = config.getConfigParameter<double>("t_initial");
     //! \ogs_file_param{prj__time_loop__processes__process__time_stepping__FixedTimeStepping__t_end}
     auto const t_end = config.getConfigParameter<double>("t_end");
+
+    //! \ogs_file_param{prj__time_loop__processes__process__time_stepping__FixedTimeStepping__n_steps}
+    auto const n_steps = config.getConfigParameterOptional<int>("n_steps");
+    if (n_steps.has_value())
+    {
+        if (t_end <= t_initial)
+        {
+            OGS_FATAL(
+                "Creating linearly spaced time steps vector using "
+                "FixedTimeStepping algorithm failed! "
+                "User provided start value (t_initial) "
+                "{} is not smaller then end value (t_end) {}.",
+                t_initial, t_end);
+        }
+
+        if (*n_steps <= 0)
+        {
+            OGS_FATAL(
+                "Requested number of time steps in time steps vector "
+                "(n_steps) must be greater then 0. "
+                "{} time steps were requested",
+                *n_steps);
+        }
+        // Create the RepeatDtPair
+        std::size_t const t_step = static_cast<std::size_t>(
+            (t_end - t_initial) / static_cast<double>(*n_steps));
+        std::vector const repeat_pairs = {
+            RepeatDtPair{static_cast<std::size_t>(*n_steps), t_step}};
+        return {t_initial, t_end, repeat_pairs};
+    }
+
     //! \ogs_file_param{prj__time_loop__processes__process__time_stepping__FixedTimeStepping__timesteps}
     auto const delta_ts_config = config.getConfigSubtree("timesteps");
 

Tests/Data/Parabolic/T/1D_dirichlet/line_60_heat.prj

@@ -53,12 +53,7 @@
                     <type>FixedTimeStepping</type>
                     <t_initial> 0.0 </t_initial>
                     <t_end> 39062500 </t_end>
-                    <timesteps>
-                        <pair>
-                            <repeat>500</repeat>
-                            <delta_t>78125</delta_t>
-                        </pair>
-                    </timesteps>
+                    <n_steps>500</n_steps>
                 </time_stepping>
             </process>
         </processes>

Tests/Data/Parabolic/T/1D_dirichlet/varying_dt/line_60_heat.xml

 <?xml version='1.0' encoding='ISO-8859-1'?>
 <OpenGeoSysProjectDiff base_file="../line_60_heat.prj">
-    <remove sel="/*/time_loop/processes/process/time_stepping/timesteps"/>
+    <remove sel="/*/time_loop/processes/process/time_stepping/n_steps"/>
     <add sel="/*/time_loop/processes/process/time_stepping">
         <timesteps>
             <pair>

Tests/Data/Parabolic/T/1D_dirichlet/varying_dt_linear/line_60_heat.xml

@@ -4,7 +4,7 @@
         <linear>true</linear>
     </add>
 
-    <remove sel="/*/time_loop/processes/process/time_stepping/timesteps"/>
+    <remove sel="/*/time_loop/processes/process/time_stepping/n_steps"/>
     <add sel="/*/time_loop/processes/process/time_stepping">
         <timesteps>
             <pair>

Tests/Data/Parabolic/T/1D_dirichlet/varying_dt_linear_compute_only_on_dt_change/line_60_heat.xml

@@ -5,7 +5,7 @@
         <linear_solver_compute_only_upon_timestep_change>true</linear_solver_compute_only_upon_timestep_change>
     </add>
 
-    <remove sel="/*/time_loop/processes/process/time_stepping/timesteps"/>
+    <remove sel="/*/time_loop/processes/process/time_stepping/n_steps"/>
     <add sel="/*/time_loop/processes/process/time_stepping">
         <timesteps>
             <pair>

web/content/docs/userguide/blocks/time_loop.md

@@ -111,6 +111,20 @@ unit:
 </time_stepping>
 ```
 
+Fixed time stepping can be also used to create linearly spaced steps between `t_initial` and `t_end`.
+The number of steps is defined by `n_steps`.
+
+```xml
+<time_stepping>
+    <type>FixedTimeStepping</type>
+    <t_initial>0</t_initial>
+    <t_end>10</t_end>
+    <n_steps>10</n_steps>
+</time_stepping>
+```
+
+will result in 11 time steps at: 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, 10.
+
 As fixed time stepping is one of the simplest available, it is a good starting point.
 
 #### Iteration number based time stepping