Fixes P2VectorToP1ScalarOperator::toMatrix()

The method was broken for 3D. We need to call getDimension() on the source
function to see whether we have a 3D setting. The destination function
represents a scalar field and, thus, we always get a 1 back.
23 jobs for kohl/toMatrix in 88 minutes (queued for 4 seconds)
Name Stage Failure
coverage Test
[ 82%] Linking CXX executable TrilinosDirectSolverTest
[ 82%] Built target TrilinosDirectSolverTest
Makefile:140: recipe for target 'all' failed
make: *** [all] Error 2
Uploading artifacts for failed job
Uploading artifacts...
WARNING: coverage/coverage.xml: no matching files
ERROR: No files to upload
Cleaning up file based variables
ERROR: Job failed: exit code 1