WIP: [ParameterTreeParser] add ParameterTreeParser for python scripts

This MR adds two static methods to ParameterTreeParser that can be used to read a python dictionary into a ParameterTree.

readPythonDict:

• Traverses all key-value pairs in the python dictionary
  • if the value is an instance of pybind11::dict readPythonDict is called for the sub-ParameterTree of the corresponding key
  • else the string representation is added to the ParameterTree for the corresponding key

readPythonFile:

• The python script is interpret using pybind11 eval_file method
• The variable with the specified name (default: __dict__, which is the global scope) is casted to a pybind11:dict and passed to readPythonDict

For readPythonDict the include files and libraries of python must be available. I added a CMake variable HAVE_PYTHON3_DEV and an entry in the config.h for that. Futhermore I added AddPythonFlags.cmake for the management of the linking and includes.

For readPythonFile the embed functionality of python must be available this is checked with the CMake variable Python3_Development.Embed_FOUND and passed to the c++ side with HAVE_PYTHON3_EMBED.

Furthermore a type_caster is added for conversion of a python dict into a ParameterTree. That makes python bindings that take a ParameterTree as a function argument callable from the python-side with a dict.

cmake/modules/AddPythonFlags.cmake

+# Defines the functions to use Python Development things
+#
+# .. cmake_function:: add_dune_python_flags
+#
+#    .. cmake_param:: targets
+#       :positional:
+#       :single:
+#       :required:
+#
+#       A list of targets to use Python with.
+#
+include_guard(GLOBAL)
+
+# set HAVE_QUADMATH for config.h
+set(HAVE_PYTHON3_DEV ${Python3_Development_FOUND})
+set(HAVE_PYTHON3_EMBED ${Python3_Development.Embed_FOUND})
+
+# register the QuadMath imported target
+if(HAVE_PYTHON3_DEV)
+  dune_register_package_flags(
+    LIBRARIES ${Python3_LIBRARIES}
+    INCLUDE_DIRS ${Python3_INCLUDE_DIRS}
+    COMPILE_DEFINITIONS "ENABLE_PYTHON3_DEV=1"
+  )
+endif()
+
+# add function to link against QuadMath::QuadMath
+function(add_dune_python3_dev_flags _targets)
+  if(HAVE_PYTHON3_DEV)
+    foreach(_target ${_targets})
+      target_link_libraries(${_target} PUBLIC ${Python3_LIBRARIES})
+      target_include_directories(dunecommon PRIVATE ${Python3_INCLUDE_DIRS})
+      target_compile_definitions(${_target} PUBLIC ENABLE_PYTHON3_DEV=1)
+    endforeach(_target ${_targets})
+  endif()
+endfunction(add_dune_python3_dev_flags)