Identify major bug in mhd_forces and force-free initial data

WARNING! Don't use the initial data functions with the --force-free
option, it doesn't work. The mhd_forces module was returning data which
mistakenly reported force balance, but it was actually just computing
the magnetic pressure gradient in two different ways. It's now giving
the correct results, with agreement between spectral and finite
difference approximations to the magnetic gradient tensor. However,
solutions to the vector Helmholtz equation del^2 B + alpha^2 B = 0 are
not generically solutions to the force balance. While the opposite is
true, something else must be done to get actual force-balanced initial
data.