implement unit-conversion

[maxnutz]

Goal

• have a stable output-set of variables, that is in sync with those used in eurostat-energy-balance_processing
• use IAMC-compatible units
• consistently map the units from pypsa-world to IAMC

To Dos:

• check, if pyam understands the units extracted from the pypsa-network
• if needed, implement mapping from pypsa output to IAMC compatible units
• create unit-conversion to nomenclature - valid units
• read units to use from definitions-input, strictly mirror behaviour in eurostat-energy-balance_processing here

[maxnutz]

currently maps pypsa-variables to IAMC-compatible units. Further investigation for understanding the energy balance units needed!