Revisit filtering with non-members of index sets

[khaeru]

#217 set up a discrepancy between ixmp_source and ixmp in whether arguments like .par(filters=...) can contain non-members of index sets. See #216 for further details.

This issue is to discuss: What should the default ixmp.Scenario/ixmp.Backend behaviour be?

1. Raise an exception if filters contains elements that are not in the respective index sets.
2. Silently discard these elements.

If consensus is for (2), no change is needed (#217 stays). If (1), #217 can be partially reverted.

@danielhuppmann @OFR-IIASA can comment.