Update course of action for stix2.1 (again)

[chisholm]

I found that two property types were wrong, for action_bin and action_reference. The former was a string property, but should be binary. The latter was also a string property, but should be external-reference. I don't know if I just goofed when I created those properties originally, or if the spec changed right after I wrote them. Seems like I wouldn't have used string properties if they weren't string typed! Maybe the spec is just changing too fast...

Also added tests for courses of action which have references (as opposed to binaries, via action_bin).

After having had the experience of writing the malware-analysis tests, I decided to change these tests to be written in a similar style. It means a lot less tedious writing of long lists of keyword args and big dicts. So the tests are better now, with less code (I hope).

[codecov-io]

Codecov Report

Merging #273 into stix2.1 will decrease coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           stix2.1     #273      +/-   ##
===========================================
- Coverage    98.64%   98.64%   -0.01%     
===========================================
  Files          119      119              
  Lines        12900    12896       -4     
===========================================
- Hits         12725    12721       -4     
  Misses         175      175

Impacted Files	Coverage Δ
stix2/v21/sdo.py	93.79% <ø> (ø)	⬆️
stix2/test/v21/test_course_of_action.py	100% <100%> (ø)	⬆️

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