Documentation of subannual timeslices erroneous

[glatterf42]

Normally, all values intended for use in the subannual category need to be registered on a platform first with ixmp.Platform.add_timeslice(). However, this is not the case for the special value Year, which indicates that the surrounding timeseries is actually valid for the whole year. This parameter is case sensitive, though, which is not always clear from the documentation. This issue, then, is supposed to fix all occurrences of year and YEAR as the special values present without prior registration.