Plasma parameters need to be tested to a precision expected by their uncertainty

Any function, or constant, that calculates, or defines, a plasma parameter should be tested to a precision related to its expected uncertainty. For example, if we have a plasma constant that is 1.5256798e-14 and it has an expected uncertainty of 2.1e-19, then the associated pytest and/or doctest should be tested to a precision of 1.5256...e-14.

All tests need to be reviewed and updated according to this scheme.

This first came to light during PR #731.