Add new facades for modeling emissions as flows

[SabineHaas]

In the sedos project we need to model emissions as flows as we are using CCS in our energy system.
Currently there's now facade in oemof.tabular handling this.

After issue #176 and discussions with @henhuy I would implement the following either in a new PR or using #170

• Add new facades ConversionWithEmissions and CommodityWithEmissions which might be children of Conversion and Commodity
• Parameters like marginal_costs will stay associated with the flows they are associated with now.
• Emission buses are added with a counter like MIMO handles buses: emission_bus_0, emission_bus_1 etc.
• emission_factors are added and processed like in mimo.csv and MIMO facade
• Implement constraints similar to conversion_factors of a converter in oemof.solph

Would be happy to hear your feedback also about the naming @Bachibouzouk & @henhuy

[Bachibouzouk]

I think the names are ok, I could only think of ConversionGHG and CommodityGHG to make the class name shorter

[SabineHaas]

I like your proposition as I find ConversionWithEmissions and ConversionWithEmissions quite long and unhandy.