MyriaUtils: better error messages from checkHasNoNulls
AppendtableTable: be a TupleTable

We need the generic functions to get numColumns, etc.
add WritableSubColumn to wrap a column of an AppendtableTable
Evaluators: switch from TupleBatch to ReadableTable

That's the real interface they expect.
Aggregators: minor cleanup w.r.t. exceptions

And schema generation
Evaluator: handle state-only expressions
BinaryExpression: fix a weird ordering issue

The test was also backwards. @domoritz am I missing something here?

When we run L < R, we want that to turn into
	left.compareTo(R) < 0. Correct?
The existing code did R.compareTo(L) > 0.
UserDefinedAggregator: initial version
UserDefinedAggregator: add tests

1) Count
2) Count with a constant output as well
3) an 'arg max'-like computation