Improve test coverage

[yalinli2]

Currently (Feb 24, 2022) only a bit over 50% of the package is covered by test, let's aim to get it up to ~80%.

See the codecov report for priorities. Adding doc examples would probably be a good solution way to deal with it as it also helps users.

I'll include our tutorial examples in the test after dyn-enabled BioSTEAM is out, hopefully that will also bump up coverage.

[yalinli2]

Also, would be good to include EXPOsan's tests (currently only run locally), include READMEs

[yalinli2]

EXPOsan's tests are now included in QSDsan

[yalinli2]

This is largely done for existing units and I'll add tests for the new units when pulling in modules from the gates branch, see #70