Fix sphere slider value when scaling coords

The value of the slider was not changed but the size of the spheres was
and when proceeding to modify the value with the slider, a glitch seemed
to appear.
Merge branch 'issue_5' into light_fixes
Merge branch 'light_fixes' into issue_11
Change preprocess_coords to be --master_pcoa aware

This function will now consider the first element of the coords
attributes the master pcoa i. e. the coordinates that serve as the
center for the ellipsoides.
Add option --master_pcoa (-s) to the interface

Passing a file with this optional option will serve to indicate the
center of the ellipsoides, passing nothing will randomly select one as
the center.