Fix Issue #868

Multiplying a scalar by a non-scalar would cause a crash in type
inference because TimeDeltaMixOp always assumed at least one of
its operands was an NPTimeDelta in its generic() method. If
neither operand is an NPTimeDelta, it now returns None so that
attempts to type the expression can proceed (and fail with a more
meaningful error message)
Merge pull request #913 from gmarkall/issue_868

Fix Issue #868