Exosims

This release marks the beginning of formal deprecation of python 2.7 support. CI now only runs on python 3.7-3.9, and new code additions will not have any python 2.7 compatibility. Subsequent releases will remove existing python 2.7-specific code, and version 3.0 will not run at all on python 2.7 and will not be guaranteed for <3.7.

This release adds:
* Utilities for querying and caching the exoplanet archive (`utils.getExoplanetArchive`)
* New forecaster-derived planet physical model as described in Savransky et al. 2019 (`PlanetPhysicalModel.ForecasterMod`)
* New `utils.planet_star_separation` method similar to `deltaMag`
* Many solar system planet phase functions and other phase functions (`utils.phaseFunctions`)
* Bug fix in `BrownCompleteness` when sample size is less than 1e8
* New planet population matching the one defined in Brown 2005 (`PlanetPopulation.Brown2005EarthLike`)
* `whichPlanetPhaseFunction` keyword in `PlanetPhysicalModel` allows selection of phase function without needing new module implementation. NB: This keyword is not included in the completeness cache files so all completeness caches will be regenerated.
* Keepout map generation instructions in the quickstart guide
* Various comment cleanups throughout the code

Re-release of 2.1.3 to synchronize with PyPI.

Bugfix release correcting extraneous `calc_dMag_per_intTime` method from Nemati_2019 optical system

Minor bugfix release correcting bad interpolation errors in Nemati_2019 optical system. All interpolants which could potentially produce (non-phyiscal) negative values with cubic splines have been changed to linear fits instead.

Note: v2.1.1 was only released on pypi and not github. It is identical to 2.1.0. Starting with this version, all releases should be automatically synchronized between github and pypi.

New features:
* New module SubtypeCompleteness which calculates the probability that a detected planet belongs to a given sub-population.
* New option to specify common inclination for whole target systems (i.e., define an ecliptic) with individual planet inclinations distributed about it. The exozodiacal cloud is assumed to be aligned with the system inclination for all individual planet calculations.
* Option to set number of exozodi for each individual target system
* New optical system: Nemati2019, implementing the calculations in the famous spreadsheet
* New method for computing stellar spectra based on Pickles atlas (spectral data is now packaged with EXOSIMS). More atlases coming in subsequent releases.

Critical Bug Fixes:
* Full compatibility with astropy 4.0+ and numpy 1.16+ Fix. Everything works in Python 2.7.8+ and 3.6/3.7/3.8.

Notes:
* Starting with this release, EXOSIMS will be available on PyPI
* Python 2 support will be dropped within the next two releases.

Acknowledgements:
This release includes significant contributions from Corey Spohn, Dean Keithly, Gabriel Soto, and Walker Dula. Thanks also to Rhonda Morgan, Mike Turmon, and Ewan Douglas.