Implement basic helper tool

[jwodder]

This PR implements most of the sample create and sample test commands described here; run them with python3 -m nwb_healthstatus sample [create|test] ....

Differences from the spec:

• I did not add an --environment option, as the description does not clearly say what it should currently do.
• test currently only outputs anything if the test method fails.
• The names of the sample files are taken from FILENAME attributes on the sample case classes (rather than from the case module basenames) in order to support multiple cases in one file.

[yarikoptic]

Great, I will merge as is but please add rudimentary setup.{py,cfg} instead of or in addition to requirements.txt

[jwodder]

@yarikoptic PR to add setup files: https://github.com/dandi/nwb-healthstatus/pull/12