Add explanation of low code-cov

[selkamand]

Robust assertion packages should have extremely high unit-test coverage

Unfortunately, because assertion function from this package are created with a function assert_create the unit tests aren't considered to cover the assertion creation statements

[selkamand]

Try and find a way to get coverage metrics that actually tell us which assertions have high/low code coverage. As it is the coverage stats are not super helpful

[selkamand]

Added a test to check a reasonable number of tests per assertion has been - don't think we can do much more than that for now. Ensure some explanation of the problem is written somewhere on the readme, then we should close

[selkamand]

Actually, a bunch of our code cov could be amplified by adding assertions on many of the lower level functions used in assertion generation. Might be worth doing this. While testing the assertion interface itself is definitely the best approach running pretty much identical tests on the the underlying functions where possible can't hurt

[selkamand]

Ok thorough review of assertions test coverage reveals that either past me was a fool or something in how test-coverage is computed has changed and our assertion creation lines don't count as towards our 'missing' test coverage - meaning that even though testing the assertions themselves doesn't get us great coverage, testing the underlying functions should get us pretty good coverage.

Going to bump up the unit testing of non-exported non-assertion functions and then close this issue