Add supporting multiple collections

[DunklesArchipel]

Add mixed_collecton, which either contains different types of entries or which consists of different types of collections.

This allows for connecting different measurement types. For example, a CVentry could have a pointer to a DEMSentry, which contains data for a measurement recorded along with the CV.

[DunklesArchipel]

partially addressed in #131.

loading now supports both pandas and tabular_resources in the collection. Depending on the operation an error is raised. other resource types can be added, and methods for those can be added accordingly.