Add kwargs to unit_eq and iterable_eq: passed on to pytest approx

[trappitsch]

This extension is useful to do unitful equality assertions while allowing the user to modify the tolerances of the approximations. Especially when dealing with instruments, such modifications can be useful.

Extended docstring to include changes.

[coveralls]

Coverage remained the same at 99.483% when pulling 05a4f8f on trappitsch:extend_unit_eq_test into 9c167d4 on Galvant:main.