Fix observations flux. Updated plotting and docs

[alexmaragko]

• Fixed spectrum.py where scaled observations would pass in fitted.py.
• Updated fitted.py y-axis units to take observations units, otherwise use database units.
• Updated savefig with bbox_inches='tight'.
• General updates to documentation.

[codecov]

Codecov Report

Attention: Patch coverage is 70.00000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 82.34%. Comparing base (22706ce) to head (3e5e39c).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
amespahdbpythonsuite/fitted.py	72.22%	5 Missing ⚠️
amespahdbpythonsuite/spectrum.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #26      +/-   ##
==========================================
+ Coverage   82.12%   82.34%   +0.22%     
==========================================
  Files          25       25              
  Lines        3743     3745       +2     
==========================================
+ Hits         3074     3084      +10     
+ Misses        669      661       -8     

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