v1.2: Updating to MIST models v1.1

This release both makes MIST version 1.1 default (notably including the new Gaia G, BP, and RP bands, as well as the TESS bandpass) and simplifies how isochrones downloads model grids. Before, it had downloaded pre-packaged grids for both Dartmouth and MIST that were gzipped tar files of selected photometric systems. Now, individual tarballs are downloaded as those bands are requested. And if you really want to use MIST 1.0 still, you may still initialize it with a version=1.0 argument. You may also try out a the MIST grids with v/vcrit = 0.4 (rather than the default non-rotating v/vcrit = 0) by initializing your MIST isochrone with the vvcrit=0.4; e.g. mist = get_ichrone('mist', vvcrit=0.4).

When you upgrade to this version, I recommend that you remove your current ~/.isochrones (or $ISOCHRONES) directory so you can start fresh and clean. This will clear out all the pre-calculated/formatted HDF grids, so you can be sure of actually using MIST 1.1 instead of reading in the old HDF files.