[WIP] cekees/set null space mprans

@ahmadia @jedbrown here is a quick attempt at fixing the problem with the null space for weak boundary conditions and zero velocity.  Once  the velocity is non-zero, the null space is empty in the current discretization, so I may need to adaptively turn this on by checking the max absolute value on the subdomain. Both 2D and 3D dambreak problems appear to be running with this (you'll need to update proteus-mprans)
This assumes the attribute is defined, it might be safer to check first that the attribute is defined, and assume not otherwise.