The mean xc bfield is calculated together with the torques as a quantity
that is needed to update the constraining fields. It is stored in the
bfield_data type.
The calculation is not a physically meaningfull thing as it ignores
the underlying mesh details, but good enough for this application.

This fixes the suprisingly small mixing needed for the constraint method
based only on the moments. As pointed out by Eduardo, comparing with the
definition in literature (e.g. Journal of Applied Physics 85, 4824 (1999)),
there was some magnetic field missing to get the update loop to the
right units (i.e. the right order of magnitude).
Tested only for bcc Fe so far, this constraint mode now works well
with a mixing factor of around 1 (0.9 is now default).