Radioactivedecay

- Add `Inventory.decay_time_series_pandas()` method to save decay data into a pandas dataframe. Time
resolution is specified by the user (104).
- Add `Inventory.decay_time_series()` method to access the decay data create by `decay_time_series_pandas()`
as a list for the time data and dictionary for the isotopic data (104).
- Drop support for end-of-life Python versions v3.6 & v3.7 (106).
- Fix `pkg_resources` deprecation warnings and some pylint warnings (106).

- Added explicit support for Python v3.12.

- Add `Inventory.to_csv()` and `InventoryHP.to_csv()` methods for writing an inventory's contents
to a CSV file (in user's chosen units) (94).

- 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).