added compile time measurement page

dev/compile-time-measurement/README.md

+Measured compilation time after changing a single file of the OGS sources.
+This should resemble the usual process development work.
+
+# Summary
+
+Compiling with fixed-size Eigen matrices takes significantly longer than with
+dynamic-sized Eigen matrices.
+
+# Used script
+
+```{sh}
+#!/bin/sh
+
+set -e
+
+export MAKEFLAGS="-j3"
+
+(
+    cd ../src
+    if [ "`git status --porcelain | wc -l`" -ne 0 ]; then
+        echo "Source directory contains untracked/uncommitted changes. Aborting" >&2
+        exit 1
+    fi
+) || exit 1
+
+
+if ! mkdir "build" &>/dev/null; then
+    echo "WARNING: reusing existing build directory!" >&2
+    read -r -p "Hit enter to continue "
+fi
+
+cd build
+
+# set some cmake option I usually use
+if ! cmake ../../src \
+    -DCMAKE_BUILD_TYPE=Release \
+    -DOGS_EIGEN_DYNAMIC_SHAPE_MATRICES=ON \
+    -DOGS_USE_LIS=ON;
+then
+    # workaround for second git worktree
+    (
+        cd CMakeFiles/git-data
+        ln -s HEAD head-ref
+    )
+    cmake .
+fi
+
+# initial build in fresh build directory
+time make ogs
+
+# make a non-trivial change to the code
+echo "inline void unused_dummy_function() {}" >> ../../src/ProcessLib/TES/TESLocalAssemblerInner.h
+
+# supposedly faster second build
+time make ogs
+```
+
+# Some results
+
+Summary: Initial compile time: ~ 10 minutes, compilation of changes only: ~ 16 seconds.
+Changing only ProcessLib/SmallDeformation/SmallDeformationFEM.h took ~20 seconds in the second build.
+
+```
+[...]
+Scanning dependencies of target ogs
+[100%] Building CXX object Applications/CLI/CMakeFiles/ogs.dir/ogs.cpp.o
+[100%] Linking CXX executable ../../bin/ogs
+[100%] Built target ogs
+
+real	9m47,808s
+user	24m2,080s
+sys	1m25,050s
+[  5%] Built target logog
+[ 10%] Built target BaseLib
+[ 19%] Built target MathLib
+[ 28%] Built target GeoLib
+[ 47%] Built target MeshLib
+[ 52%] Built target MeshGeoToolsLib
+[ 61%] Built target NumLib
+[ 75%] Built target MaterialLib
+Scanning dependencies of target ProcessLib
+[ 75%] Building CXX object ProcessLib/CMakeFiles/ProcessLib.dir/TES/CreateTESProcess.cpp.o
+[ 75%] Building CXX object ProcessLib/CMakeFiles/ProcessLib.dir/TES/TESLocalAssemblerInner.cpp.o
+[ 77%] Building CXX object ProcessLib/CMakeFiles/ProcessLib.dir/TES/TESProcess.cpp.o
+[ 77%] Building CXX object ProcessLib/CMakeFiles/ProcessLib.dir/TES/TESReactionAdaptor.cpp.o
+[ 78%] Linking CXX static library ../lib/libProcessLib.a
+[100%] Built target ProcessLib
+[100%] Built target ApplicationsLib
+[100%] Linking CXX executable ../../bin/ogs
+[100%] Built target ogs
+
+real	0m15,544s
+user	0m26,212s
+sys	0m1,759s
+```
+
+## For reference: fixed-size Eigen matrices, three threads, ccache not disabled (but maybe not effective)
+
+initial build:
+```
+real	55m52,166s
+user	63m0,470s
+sys	7m32,749s
+```
+
+build after change:
+```
+real	2m6,759s
+user	2m11,119s
+sys	0m2,841s
+```