Changing database schema about orion

[kveerama]

@csala's comment:
About the Experiments: at some point we discussed a slightly different schema, that would be as follows:

• A Dataset is associated with a Satellite and has a collection of Signals
• A Signal is associated with a single CSV file and has a start and a stop.
• An Experiment is related to a Pipeline and a Dataset.
• Dataruns are related to an Experiment and a Signal, which implicitly relates them to a Dataset and a Pipeline.

For this, we should:

• Rename the current Dataset to Signal.
• Remove the satellite_id and signal_set fields from the Signal model.
• Add another Dataset model that is parent to Signal and has the satellite_id field.
• Create the Experiment model and make it related to Dataset and Pipeline.
• Make the Dataruns related to the Experiment.
• Remove the Datarun <-> Pipeline relationship (as it comes through the Experiment).

Can we adapt the changes to this schema?

[AlexanderGeiger]

Closed in #58