Don't assume MKS constants
Don't assume MKS constants

Though the unit tests should catch this if it ever changed, it's a bit of a pointlessly dangerous asssumption.

Co-authored-by: Marten van Kerkwijk <38b3863dd0bf5d8cd259bfed3b9cb0c897ac8000@astro.utoronto.ca>
Signed-off-by: Nathaniel Starkman (@nstarman) <ac14c573519de521a9fc5a84607d51c0c956190b@protonmail.com>

astropy/astropy,larrybradley/astropy,astropy/astropy,saimn/astropy,lpsinger/astropy,StuartLittlefair/astropy,astropy/astropy,saimn/astropy,saimn/astropy,lpsinger/astropy,StuartLittlefair/astropy,StuartLittlefair/astropy,mhvk/astropy,larrybradley/astropy,mhvk/astropy,pllim/astropy,pllim/astropy,saimn/astropy,pllim/astropy,mhvk/astropy,lpsinger/astropy,lpsinger/astropy,astropy/astropy,astropy/astropy,mhvk/astropy,saimn/astropy,larrybradley/astropy,pllim/astropy,pllim/astropy,StuartLittlefair/astropy,mhvk/astropy,StuartLittlefair/astropy,larrybradley/astropy,larrybradley/astropy,lpsinger/astropy