#388 Truncate Sersic profiles


Calculate scale radius b for truncated Sersic
Flux normalization
FT (Hankel transform) caching
Photon shooting set-up
Caching of Sersic information by (n, maxRre) pair (where maxRre == trunc / re, and resolution of maxRre is limited to 2 decimal places)
Add in-code comments
Added a line in CHANGELOG.md about Sersic truncation
I believe I need to add

Unit test(s) [I will look at test_SBProfile.py, test_GSObject.py, and test_InterpolatedImage.py]
A way to add this to the config format (uggg)
Demo (?)