WIP: Evolutionary tracks for physicsgrid

[karllark]

Adding evolutionary tracks in as an option. Joins the current only option of isochrones.
The idea is that for single star SED fitting, evolutionary tracks may may allow for a better sampling of the physical parameter space with fewer points. To be determined.

Closes #267.
Closes #18.

Manual rebase from PR #284 (too many conflicts to auto rebase easily).

[coveralls]

Coverage decreased (-0.6%) to 42.027% when pulling 0cf679c on karllark:add_evol_tracks into 21e363b on BEAST-Fitting:master.

[lgtm-com]

This pull request introduces 1 alert when merging 28616e4 into 21e363b - view on LGTM.com

new alerts:

• 1 for Missing call to __init__ during object initialization

[lgtm-com]

This pull request introduces 1 alert when merging 0cf679c into 81d5080 - view on LGTM.com

new alerts:

• 1 for Unused local variable

[karllark]

Getting this working again.

Recovered how to generate the MIST fits files from the ascii files downloaded from the MIST website (https://waps.cfa.harvard.edu/MIST/model_grids.html#eeps).

Update the evolutionary track plots to give more information about the regridding. An example in the next comment.

[karllark]

MIST evoltracks for feh=p0.00. Black is as downloaded. Blue is regridded to uniform logM and condensed with to remove points w/ less than the deltas given as computed along a single mass evolutionary track.

(base) [kgordon@dirty MIST]$ python ~/Python/beast/beast/plotting/plot_evoltracks.py MIST_FeH0.00.fits --condense 
size orig =  259827
size regrid =  14719
logM range: [-1.0, 3.0]
logM: 0.05
condense: True
condense deltas
logT: 0.05
logL: 0.05
grid metrics: diffs [min,  max, median, mean]
logL
  orig: [3.435018935959988e-10, 0.2862267761637278, 0.008897551923432001, 0.013521154589942302]
regrid: [5.578930625471656e-06, 0.23607140365266965, 0.03816796642929332, 0.03599822784774602]
logT
  orig: [9.128076072784097e-11, 0.29449549130961605, 0.0018497452251660107, 0.0029010444414949966]
regrid: [2.6497791516533198e-08, 0.3071999709304931, 0.00470920031562283, 0.010666433098119679]
logg
  orig: [4.949631815520661e-10, 1.0909475227034098, 0.015412822012143668, 0.02205363698970596]
regrid: [1.2148029982561592e-05, 1.1488386623502533, 0.05239822156450702, 0.06607721382890888]