Improve IamSlice feature

[danielhuppmann]

As a follow-up to #657 per a suggestion by @coroa:

The next step for the IamSlice would be to refactor the second half of IamDataFrame.filter into a new method (could be called subset for instance) or generic helper and use that from getitem and filter; so that idf[idf.slice(**kwargs)] is equivalent to idf.filter(**kwargs).

(Currently getitem is dropping meta and does not do the year<->time swapping correctly)