add the configuration to the datatree
in order to reproduce computation results, we want to output the config that was used to produce that result. This output should include also default values that where used and one should be able to use it directly to redo a certain computation. This commit adds a function template `get_and_set` which wraps the behavior of `ParameterTree::get`. Besides retrieving the value from the `ParameterTree`, it also writes the value to the given `DataTree`.