Make sure Selection handles NaN and NullData

[dlindhol]

The value pattern match at line 58 of Selection.scala is not exhaustive. It does not have a case for Some(Data) which is where NullData would fall.

It's also not clear how ordering works for NaNs (in light of TotalOrdering vs IeeeOdering).

I'm thinking that we should use things like Ordering.gt instead of using our matches on the result of a tryCompare. The ordering test should always be false for NaN and NullData.

[dlindhol]

Substitution also has a non-exhaustive match. The return to NullData as a possibility for Scalars instead of NullDatum has made this a bigger issue.