Add more matrix operations, with an eye on adouble types

This patch adds several more implementations of operator+ etc for FieldMatrices.
In particular, you can now combine matrices of adoubles (the 'active' type from
the ADOL-C automatic differentiation library) with regular double types.

These new operators are needed for the NonplanarCosseratShellEnergy class,
which is coming in the next patch.