More spark

Added an op to the expr level, but mostly stuff for spark compute:

- '!=' now in for selection expressions on TableExpr
- Map for spark
- Distinct for spark
Hrm, this is really a test for `Selection`, not a test for `NE`.  Probably `NE` tests should go elsewhere.  If NE were to break then we might check the title of the test and assume that something was wrong with `Selection`