Outdated docs for use of multi-periods

[henhuy]

In the multi-period section in the docs it's stated that you can parametrize the periods as dict:

oemof-solph/docs/usage.rst

Lines 1027 to 1035 in f02297e

	import pandas as pd
	import oemof.solph as solph
	my_index = pd.date_range('1/1/2013', periods=17520, freq='H')
	periods = {
	0: pd.date_range('1/1/2013', periods=8760, freq='H'),
	1: pd.date_range('1/1/2014', periods=8760, freq='H'),
	}
	my_energysystem = solph.EnergySystem(timeindex=my_index, periods=periods)

This seems to be outdated (I think a list should be used instead), as I get error AttributeError: 'int' object has no attribute 'min' in line

oemof-solph/src/oemof/solph/_energy_system.py

Line 192 in f02297e

periods_years.append(v.min().year - start_year)

[jokochems]

Thanks, @henhuy. I'll provide a fix shortly. If you have time, feel free to already do so.