Update module structure

[ClaraBuettner]

This branch slightly adjusts the structure of etrago. There are now more submodules, and especially the tools module is now smaller. The different modules are quite similar to the flow-chart we often use to show the functionalities of eTraGo, which makes it easier to combine both. The structure now looks like this:

• etrago
• etrago.appl
• etrago.network
• etrago.analyze -- etrago.analyze.calc_results -- etrago.analyze.plot
• etrago.cluster -- etrago.cluster.electrical -- etrago.cluster.gas -- etrago.cluster.snaphot -- etrago.cluster.spatial
• etrago.disaggregate -- etrago.disaggregate.spatial -- etrago.disaggregate.temporal
• etrago.execute -- etrago.execute.grid_optimization -- etrago.execute.market_optimization -- etrago.execute.sclopf will be added
• etrago.tools -- etrago.tools.io --- etrago.tools.io.db -- etrago.tools.constraints -- etrago.tools.extendable -- etrago.tools.utilities

I am not sure if that is the very best structure but it is in my opinion already an improvement to the modules before, where many things were in the tools module. It might make sense to move e.g. constraints and extendable to the execute module. We can discuss the structure. This setting is already working, the tool itself and als the rtd is working fine.

[ClaraBuettner]

@ulfmueller: Do you have any thoughts/concerns on this?