Implement representative years

[nailend]

This is the section to implement the initialization of representative years. Add the flag use_representative_years.

True or False may be derived from stepsize in periods.years>1

Solph branch features/adapt-costs-for-multi-year-periods needs to be selected.

Might not be possible to merge this to dev, as solph branch will not be merged into dev, and therefore might be a feature which can lead to erros due to passing the parameter use_representative_years to energy_system if not existent.

[SabineHaas]

Needed solph branch is
features/adapt-costs-for-multi-year-periods
see oemof/oemof-solph#993