Feature: Modify the source used to gather a specific metric

[EndzeitBegins]

Evaluation on how to implement this feature is ongoing. See issue #15.

Not scheduled for the initial release.

[Laberbear]

On the gathering side the implementation is pretty trivial.

Each time a measurement is taken the list of measuring sources is iterated and the first that is able to deliver the data is used.
A simple dictionary in the data_manager containing the name of source that should be used for each metric could be checked beforehand.