Enables assignment across different DenseMatrix implementations, e.g. assignment from a DynamicMatrix to a FieldMatrix (this previously either compiled and segfaulted or did not compile)
Catches invalid FieldMatrix assignments at compile-time
Retaines the DenseMatrixAssigner class in order to remain compatible with user-defined matrix classes
To me it looks 99% good to go. There is only remaining question that I cannot answer on my own:
Should FieldMatrix<double, 3, 3> x = 27 compile? In other words, should the constructor be marked as explicit or not? Of course, one can always use FieldMatrix<double, 3, 3> x(27) instead.
I've tested this fix with multiple modules and it doesn't break anything for me.
