Filters in the facade layer

[danielhuppmann]

When working with pyam over the past week, I noticed the following missing filter options in the facade:

• For Platform.iamc.tabulate():
  There is a shorthand to filter by year (which converts to step_year) which takes an integer but not a list of integers. The corresponding "simple" filters like region take a string or a list of strings.

• For Run.iamc.tabulate():
  It is not possible to filter by year or step_year, plus a shorthand-filter like year=[2005, 2010] would be nice.

[glatterf42]

Maybe #208 should introduce a year: HasYearFilter (which doesn't exist yet) instead of just introducing year__gte.