Fixed a bug in constraint bfield calculation related to muffin tins.

The muffin tin index was taken from the old mesh, which is not correct
and can even give errors if it is larger than the total size in the
new mesh.

Introduced a function in the Chebyshev mesh module that calculates
the index of the muffin tin radius. Formula for this is taken from
KKRhost.
Used this function where before the muffin tin index of the old mesh
was used. Added the dependency on this function to the Makefile.