Allow constant null vector in multigrid_invert_test if no --load-vec= files are specified.  Used for testing free field.
Merge branch 'multigrid' of https://github.com/lattice/quda into multigrid

Conflicts:
	include/ks_improved_force.h
	tests/invert_test.cpp
Fixed merge of invert_test.cpp - uses free field mass.
Merge branch 'multigrid' of https://github.com/lattice/quda into multigrid
Commented out profiler in inv_bicgstab.cpp to prevent multiple timers error in MG V-cycle.

Fixed the calculation of the residual after post-smoothing.