Naive maps updates

[petiay]

Added several options:

• Can now calculate the median of the BEAST results (instead of by default just the mean)
• Can make maps of three stellar parameters - for now they are log(T), log(age), and initial Mass. Easy to tweak or add more.
• Can input a chi2min threshold to cut BEAST stats before making maps
• Updated map file naming (from experience it's nice to know the map resolution at a glance esp. when there are comparisons with various ISM maps).

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 5.88235% with 16 lines in your changes missing coverage. Please review.

Project coverage is 14.75%. Comparing base (8d43cf9) to head (0b512d8). Report is 1 commits behind head on master.

Files	Patch %	Lines
megabeast/tools/make_naive_maps.py	5.88%	16 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #71 +/- ## ========================================== - Coverage 15.21% 14.75% -0.46% ========================================== Files 6 6 Lines 355 366 +11 ========================================== Hits 54 54 - Misses 301 312 +11 ```

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[karllark]

No tests to speak of for megabeast yet. So ignoring these for now.