Version 2.1 released

[Mike Jarvis]

We are pleased to announce the release of GalSim version 2.1.  

In most cases, you can get the new version by running 

pip install -U galsim

or 

pip install galsim==2.1

Most of the changes in version 2.1 involve efficiency improvements in running the photon shooting rendering method.  Especially using PhaseScreenPSFs. These changes were particularly important for the LSST DESC DC2 simulation runs using ImSim.

Deprecated Features:

- Deprecated PhaseScreenPSF attributes `img` and `finalized`.  These are now implementation details, not part of the public API. (#990)
- Deprecated GSParams items `allowed_flux_variation`, `small_fraction_of_flux`, and `range_division_for_extreama`.  These are no longer used. (#993)

New Features

- Added ability for RandomWalk profile to follow any arbitrary profile (any GSObject that can be photon shot) rather than just a Gaussian. (#821)
- Added spline as LookupTable2D interpolant. (#982)
- Added ability to use a galsim.Interpolant for LookupTable and LookupTable2D interpolants. (#982)
- Added option for faster grid interpolation of LookupTable2D. (#982)
- Added optional `offset` and `flux_ratio` parameters to `WCS.toWorld` and `WCS.toImage` functions. (#993)

Bug Fixes

- Updated the diffusion functional form for the Silicon class to match lab experiments of ITL sensors. (#981)
- Fixed a bug in the PhaseScreenPSF `withGSParams` function when using geometric_shooting=True.  It had been erroneously preparing the screens
  for FFT rendering, which was both incorrect and slow. (#990)