Add xAXPY
Fix MAGMA inclusion and some documentation errors

Removed the magma_lapack.h header, which seems to be necessary only if
there is no other lapack header included.
Various smaller things have been fixed in the documentation.
Add matrix addition
Add submatrix clone and templated allocate_like()
Add Fills::lapack_rand and Fills::unity

To start with, lapack_rand is probably ok. In good C++ spirit I should
probably add a random filler that takes an std::function to fill the
matrix some time later.
Fix type casting with MAGMA
Add a test to check for matrix dimension matches
Fix various minor things