Error in first code example in user's guide

[jankoc]

There seems to be a bug in the first code example in the user's guide:

from oemof.solph import create_year_index
my_index = create_year_index(2011)

After 10 minutes of searching I found out that "create_year_index" seems to have been replaced by "create_time_index". This should be fixed as it can be enough to scare off potential new users.

[p-snft]

Thanks for bringing this up. This is the kind of issues that experienced users just overlook.