add detailed docstrings for pmag.dir2cart, pmag.fisher_mean, dobingham
add detailed docstrings for pmag.dir2cart, pmag.fisher_mean, dobingham

lfairchild/PmagPy,lfairchild/PmagPy,Caoimhinmg/PmagPy,Caoimhinmg/PmagPy,Caoimhinmg/PmagPy,lfairchild/PmagPy