Serialize energysystem to datapackage

[jnnr]

This is a draft for a function that takes an oemof.solph.EnergySytem and writes a datapackage. Maybe this is interesting for you, @Bachibouzouk.

The idea is to map oemof.solph's parameters to a data model where each type of component has its own table. Type of components means classes like Sink, Source, Transformer, GenericStorage, with the addition that the number of outputs and inputs is distinguished. A Transformer in particular can have n inputs and m outputs, in general.

(Component, None): var_name maps to (var_name, none, none)
(bus, component): var_name maps to (var_name, inputs, bus_n)
(component, bus): var_name maps to (var_name, outputs, bus_n)

This leads to tables like this (GenericStorage 1 input 1 output):

--	--	--	--	--	--	--	--	--
var_name	label	storage_capacity	bus_1	bus_2	nominal_value	nominal_value	variable_costs	variable_costs
direction	None	None	inputs	outputs	inputs	outputs	inputs	outputs
connection	None	None	None		bus_1	bus_2	bus_1	bus_2
	storage-1	100	gas	gas	1	1	0	22
	storage-2	22	electricity-1	electricity-1	1	1	0	22
	storage-3	120	electricity-2	electricity-2	1	1	0	22

The output format can in a next step be mapped to oemof.tabular. E.g. variable_costs on inputs would be carrier_costs by tabulars convention, variable_costs on outputs would be marginal_costs, so the multiindex columns would map to something like:

l	label	storage_capacity	from_bus	to_bus	from_capacity	to_capacity	carrier_cost	marginal_cost

In another step, tabular's facades' mapping to solph in build_solph_components could be simplified.
Some facades are quite generic: Storage, Conversion, Load, Link.
Facades with several nodes are not invertible.