[Doc] documented process variables

Documentation/ProjectFile/prj/process_variables/i_process_variables.md

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+Contains definitions of process variables.

Documentation/ProjectFile/prj/process_variables/process_variable/boundary_conditions/i_boundary_conditions.md

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+Contains boundary conditions for the process variable.

Documentation/ProjectFile/prj/process_variables/process_variable/boundary_conditions/t_boundary_condition.md

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+Specifies a single boundary condition.
+
+See \ref ogs_file_param__boundary_condition.

Documentation/ProjectFile/prj/process_variables/process_variable/i_process_variable.md

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+Defines a single process variable.
+
+A process variable is a primary variable for which some process is solved. For a
+process variable initial and boundary conditions can be specified.

Documentation/ProjectFile/prj/process_variables/process_variable/t_components.md

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+Sets the number of components of the process variable.
+
+It is one for scalar variables and greater than one for vectorial (or
+theoretically even tensorial) process variables.

Documentation/ProjectFile/prj/process_variables/process_variable/t_initial_condition.md

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+Initial condition for the process variable.
+
+This setting references a \ref ogs_file_param__parameter "parameter" by name.

Documentation/ProjectFile/prj/process_variables/process_variable/t_name.md

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+Name under which the process variable can be referenced.