Allow some DenseMatrix operations with mixed types

[roystgnr]

This needs a little testing before merge.

[moosebuild]

Results of testing fe0a40a using libmesh recipe:

Failed on: linux

View the results here: https://www.moosebuild.com/view_job/1817

[jwpeterson]

FWIW, here's the error

./include/libmesh/fe_base.h:75:8: error: redefinition of 'struct libMesh::TypesEqual<T1, T2>'
./include/libmesh/compare_types.h:45:8: error: previous definition of 'struct libMesh::TypesEqual<T1, T2>'
./include/libmesh/fe_base.h:80:8: error: redefinition of 'struct libMesh::TypesEqual<T, T>'
./include/libmesh/compare_types.h:50:8: error: previous definition of 'struct libMesh::TypesEqual<T, T>'

[moosebuild]

Results of testing 26897e8 using libmesh recipe:

Passed on: linux

View the results here: https://www.moosebuild.com/view_job/1824