Implement creation and loading of collections

[kwerber]

Is your feature request related to a problem? Please describe.

Currently, Trufos is only able to load the default collection. Since we want users to be able to create and load other collections, we need to implement the necessary functionality in our backend persistence service.

Describe the solution you'd like

The persistence service should provide the necessary methods to...

• load a collection at some given file path
• create a new collection at a given file path

Additional context

The way we currently designed things, Trufos can only ever have one collection open at a time. This means when loading a collection any in-memory state or data for the old collection needs to be replaced. When working on this issue, make sure to also write unit tests for this new functionality.