more expressive matrix method names
A problem was recently encountered by @philipc while porting some foam-extend code.
In the OpenFOAM Matrix class, we use m() for rows and n() for cols, whereas foam-extend use the opposite naming.
This obviously makes for some very difficult error-finding.
Since there is nothing intrinsically meaningful about m and n, I suggest that we use rows() and cols() instead (method names as per Eigen3) and mark m() and n() as deprecated. This would help identify these types of pitfalls in the future.