#388 Truncate Sersic profiles

This pull request is for Sersic class modification, which allows for truncation at a certain radius.  The added features are:

- Truncation of Sersic profiles, including
  - Calculation of 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
- Add a line in CHANGELOG.md about Sersic truncation
- Unit tests
- Config format + documentation update

There is no truncated Sersic examples included in the demos, but hopefully the documentation will suffice.