L2ProjectionInverseDiscretizationConnection should allow modal output

This could avoid the forced (and possibly un-needed) modal-to-nodal conversion at the end of the connection:

https://github.com/inducer/meshmode/blob/cdcc7f5e4ae883a31ac9da3629b0acd2289e2a34/meshmode/discretization/connection/projection.py#L219-L229

I *think* this is easy to do since it receives a connection as input. It just needs to check whether the output side of the connection is modal, and if so, skip the extra conversion to nodal.

I noticed this while reviewing #192.

cc @alexfikl @thomasgibson