Merge branch 'feature/force-installation-of-python-modules' into 'master'

[cmake][python] use `pip --upgrade` on install

Closes #81

See merge request core/dune-common!422

cmake/modules/DunePythonInstallPackage.cmake

@@ -79,7 +79,7 @@ function(dune_python_install_package)
   endif()
 
   set(INSTALL_CMDLINE -m pip install
-                      "${INSTALL_OPTION}" "${WHEEL_OPTION}" "${EDIT_OPTION}" ${PYINST_ADDITIONAL_PIP_PARAMS}
+                      "${INSTALL_OPTION}" --upgrade "${WHEEL_OPTION}" "${EDIT_OPTION}" ${PYINST_ADDITIONAL_PIP_PARAMS}
                       "${PYINST_FULLPATH}")