Synonym-dict

Ignore 'parent': None; detect missing parent compartments

Added a new testcase for merging nonstandard CAS. Removed some verbose output from `CompartmentManager`

Change approach to dealing with nonstandard CAS numbers. On an input of '0034-56-7', the non-standard
entry (which still matches the regexp) should be accepted as a synonym, though other possible
defective entries are not. Inspired by defects observed in ecoinvent 'Naphtalene' _(sic)_.

Also renamed `compartments.compartment.NullContext` (exception) to `NullContextException`.

Fix error with improper adding of query terms to reverse-lookup dictionary (issue 6).

Wrap-up release that encapsulates improvements with "non-specific" compartments and performance improvements.
* Anything named "xxx, unspecified" is no longer considered distinct from the thing named "xxx" (close 4)
* Tuples are now acknowledged as the "proper" way to reference a compartment
* Major performance improvement when testing SynonymSet membership (0.1.6.post0)
* Upgrade "example" cases to full-blown subclasses and improve exports

Nonspecific child compartments submitted to `CompartmentManager.__getitem__` can be bypassed.
I think we are getting close to 0.2.0 here!

- `post0` (2021-02-24) Major performance improvement on `SynonymSet.__contains__`