timothydmorton/isochrones: Version 2.0 --- major update

Isochrones 2.0 contains the following major updates and improvements: All-new documentation: https://isochrones.readthedocs.io Major re-implementation of most of the core objects. Grids are now interpolated using "equivalent evolutionary phase," or EEP, which enables fitting evolved stars. Two kinds of interpolation grids may be used; either isochrone grids, for which the grid parameters are (EEP, age, [Fe/H]), or evolution track grids, for which the grid parameters are (mass, EEP, [Fe/H]). Evolutionary model grid interpolation and bolometric correction interpolation are explicitly separated. The use of bolometric correction grids also means that treatment of extinction is much more accurate than in isochrones v1.x. Addition of a new .generate() method for the model grids that explicitly allows generation of synthetic stellar properties, as a function of (mass, age, [Fe/H])---even though the properties are interpolated using EEP. New, faster-fitting StarModel objects for simple specialized fitting cases (i.e. single stars and fully blended multiples): SingleStarModel, BinaryStarModel, and TripleStarModel. Note that isochrones 2.0 currently implements only the MIST models (and the MIST bolometric correction grids). But hopefully we can incorporate other new models grids soon