Add extract description in docs

[DrDomenicoMarson]

Just a couple of things that I forgot about:

• added the description of extract to the main parsers page in the documentation
• the test in test_namd.py now properly tests if 'dHdl' is not set when calling extract

[codecov[bot]]

Codecov Report

Merging #243 (3ae4bd6) into master (48a719b) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #243   +/-   ##
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  Coverage   98.56%   98.56%           
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  Files          25       25           
  Lines        1672     1672           
  Branches      353      353           
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  Hits         1648     1648           
  Misses          3        3           
  Partials       21       21           

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