Radioactivedecay

- Add `rd.read_csv()` function for creating an inventory by reading nuclides & quantities (and
optionally units) in from a CSV file (94).

- Fix bug whereby inventories would not instantiate with NumPy int datatypes (e.g. `numpy.int32`)
for the nuclide quantity in the instantiation dictionary. Previously a ValueError was raised (96).

- Prohibit instantiation of an inventory if the user supplies an activity for a stable nuclide
(92).

- Fix bug where the conversion of dpm to other activity units, and vice versa, was incorrect (87 &
88).

- Added support for Python v3.11. Use latest importlib-resources API (`files()` etc.) to fix
warnings. Fix pylint `__repr__()` usage warnings in tests. Use isort to sort imports and check in
code formatting action (85).

- Fix normal precision inventory unit tests for `decay()` and `cumulative_decays()` methods (84).
The tests now warn if the calculated floats are not exact matches for the test defaults, and assert
that the calculated floats are within max(rel_tol=1e-7, abs_tol=1e-30) of the expected values. This
means the tests that had previously started flaking on GitHub Actions CI (sporadically) will now
pass.