The freva core should be a REST API

There are a couple of reasons why this might be useful:

• We do have a unified entry point for the freva core behaviour and no potentially circular behaviour.
• We can very simply provide freva functionalities in any kind of programming/scripting language. Because any language supports rest calls.
• The current python client will be much smaller.

The first action times that need to be undertaken are:

• Setting up a databrowser REST API
• Setting up a history REST API
• Disentangle the freva plugin API
• Delete all redundant code on the freva core.