add dens_in_gevcc
better physical conversions decorator
option to pop ro and vo
update acknowledgements in README and docs
provude a little more documentation
use functools.wraps to properly transfer name and documentation of decorated functions; add galcenrect_to_XYZ to documentation; fixes #164
add test for #163, fails currently
deal with incorrect input of XYZ=True and degree=True, fixes #163