Split metadata YAML file up into one file per question collection

One big file of metadata is just too big to be handy. We would like to split it up into one file per question collection. This becomes handy as well when we are doing more surveys, so that we can re-use question collections and therefore also whole files.

The logic needs to iterate over all files in a metadata folder and create one big and comprehensive metadata data structure.

We need to adapt the logic for configuring the framework with a configuration YAML file and adapt the documentation as well.

Related to #61 (closed).