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`.