Move functionality to oemof.tabular

[jnnr]

The generic functionality here can move step-by-step to oemof.tabular. If most or all can be moved, this would make maintenance easier.

These can be addressed one after the other:

• Postprocessing (after Refactor postprocessing #82 is merged and tested in practice for a bit)
• Plots

These belong together

• DataFramepackage class
• Define schemas for tabular data packages that describe energy systems in facade_attrs and the available components and busses in component_attrs.yml and busses.yml
• Setup datapackages by passing the lists of regions, links, busses and components to EnergyDataPackage.setup_default. Custom-defined busses and components can be passed as dictionaries bus_attrs_update and component_attrs_update.
• Parametrize the EnergyDataPackage with EnergyDataPackage.parametrize
• stacking, unstacking
• sensitivity

This could be checked, too:

• Infer metadata (mainly done by oemof.tabular already, but maybe there are some extra features that can improve tabular).

[jnnr]

Remember to clean up oemoflex: Remove functionality that has been moved or is not used any more.