NF - added signal-to-noise function to stats
Changed function name of SNR to signal_to_noise
changed function name and cleaned up some grammar
added gain parameter and made suggested changes to documentation
removed gain and fixed some errors
Added gain and updated documentation
added simple test for snr function
added more tests for snr function
added Msun alias for solar mass unit
Remove unneeded continuation marks somebody added to one of the imports.
Simplify conversion to np.float for input list
No longer try to preserve data type of input array if it is not
some type of floating point.
Update for astrofrog's comments
Integer+fractional day implementation, using double-doubles.
Tests adapted as well.
Add iterative wcs.all_world2pix

Iterative inverse of wcs.all_pix2world. Closes #1066.
Add tolerance option for wcs.all_world2pix
Add unit test for wcs.all_world2pix
Rename pixes to pix

The name of this variable is meant to echo the name of the function,
world2pix.
Make `Angle` a subclass of `Quantity`.
Python 3.x fixes