Merge branch 'boyden_spinmixing' into develop

This introduces the following options to the KKRhost code:

* NSIMPLEMIXFIRST= integer: number of simple mixing steps that are done before Broyden or Anderson mixing starts
* SPINMIXQBOUND= real: threshold of the rms of the nonco angles (in degrees) that is used to fix the directions if the angles change less than this value
* <use_boyden_spinmix>= boolean: activate broyden mixing for noncollinear angles
* <write_angles_alliter>= boolean: activate writeout of the nonco angles for all iterations in a new file

The following parameter control the new broyden mixing for the nonco angles:

* SPINMIXALPHA= real: mixing parameter use in the Broyden spinmixing
* SPINMIXNSIMPLE= integer: number of simple mixing steps before Broyden spin mixing starts
* SPINMIXMEMLEN= integer: number of iterations that are remembered in the Broyden mixing