Add and test minimum requirements for PyNWB

[rly]

Addresses #1147.

• Add requirements-min.txt and make setup.py read from that
• Add CI tests with the minimum requirements
• Update documentation regarding the requirements files

[codecov]

Codecov Report

Merging #1148 into dev will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##              dev    #1148   +/-   ##
=======================================
  Coverage   68.09%   68.09%           
=======================================
  Files          37       37           
  Lines        2282     2282           
  Branches      398      398           
=======================================
  Hits         1554     1554           
  Misses        659      659           
  Partials       69       69

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