Mpan

* Allow equality comparison with strings.

* We rewrote the distribution portion of the parser to better reflect the n:1
relationship between GSP groups and distributors. The `.gsp_group_id`
(`str`) is gone, replaced with `.gsp_groups` (`list[str]`).
* The API for DNOs and IDNOs are now the same, dropping properties like
`.licensee` and `mpas_operator_id` in favour of a common `participant_id`.
See the docs for specifics.

* Added support for automatic generation of valid MPANs with either Faker or
Mimesis.

* Minor update to the validation error message.

* Bugfix: Comparing two identical MPAN objects now returns boolean `True`,
while comparing an MPAN object to a string of the same value returns `False`.
* Added lots more documentation to the README.

* `.is_valid()` was amended to validate the top row as well.
* `is_valid()` now returns a boolean rather than potentially throwing an
`InvalidMPANError`.