Enable pickling of Mahalanobis measure

[sdhiscocks]

Fixes #748

[codecov[bot]]

Codecov Report

Base: 94.92% // Head: 94.94% // Increases project coverage by +0.01% :tada:

Coverage data is based on head (07dff21) compared to base (18ee65a). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #750 +/- ## ========================================== + Coverage 94.92% 94.94% +0.01% ========================================== Files 171 171 Lines 8454 8464 +10 Branches 1232 1233 +1 ========================================== + Hits 8025 8036 +11 Misses 317 317 + Partials 112 111 -1 ``` | Flag | Coverage Δ | | |---|---|---| | integration | `69.75% <80.00%> (-0.03%)` | :arrow_down: | | unittests | `92.06% <100.00%> (+0.02%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dstl#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://codecov.io/gh/dstl/Stone-Soup/pull/750?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dstl) | Coverage Δ | | |---|---|---| | [stonesoup/measures.py](https://codecov.io/gh/dstl/Stone-Soup/pull/750/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dstl#diff-c3RvbmVzb3VwL21lYXN1cmVzLnB5) | `95.68% <100.00%> (+1.35%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dstl). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=dstl)

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