Add simulation code

[karllark]

Move the existing simulation code from the beast and enhance it to use the megabeast capabilities to update the prior model on the fly (instead of needing to recompute the full physicsmodel).

Partially addesses #76
Closes #66

[codecov]

Codecov Report

Attention: Patch coverage is 19.75806% with 199 lines in your changes missing coverage. Please review.

Project coverage is 19.18%. Comparing base (a9348c4) to head (a51f1c6).
Report is 19 commits behind head on master.

Files with missing lines	Patch %	Lines
megabeast/tools/simulate_obs.py	9.78%	166 Missing ⚠️
megabeast/helpers.py	17.64%	28 Missing ⚠️
megabeast/mbmodel.py	88.46%	3 Missing ⚠️
megabeast/fit_single.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #77      +/-   ##
==========================================
+ Coverage   14.43%   19.18%   +4.74%     
==========================================
  Files           6        6              
  Lines         374      563     +189     
==========================================
+ Hits           54      108      +54     
- Misses        320      455     +135     

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