feat: export database as dataclass for easier access

[tsutterley]

docs: update getting started and README.rst
test: add test for reading model database
chore: bump version to 2.1.6

[codecov]

Codecov Report

Attention: Patch coverage is 92.00000% with 2 lines in your changes missing coverage. Please review.

Project coverage is 72.57%. Comparing base (bd3d702) to head (1716537).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
pyTMD/io/model.py	86.66%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #337      +/-   ##
==========================================
+ Coverage   71.65%   72.57%   +0.92%     
==========================================
  Files          39       39              
  Lines       11042    10291     -751     
  Branches     1522     1370     -152     
==========================================
- Hits         7912     7469     -443     
+ Misses       2632     2358     -274     
+ Partials      498      464      -34     

Flag	Coverage Δ
unittests	72.57% <92.00%> (+0.92%)	⬆️

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