Make `switch()` statements error when there's no match

[jamesmbaazam]

• [x] I have read the CONTRIBUTING guidelines

• [x] The commit message follows our guidelines

• [x] Tests for the changes have been added (for bug fixes / features)

• [ ] Docs have been added / updated (for bug fixes / features)

• What kind of change does this PR introduce? (Bug fix, feature, docs update, ...)

A failsafe to prevent bugs.

• What is the current behaviour? (You can also link to an open issue here)

The helper functions .update_chain_stat() and .get_statistic_func() used switch() statements that did not error if there was no match. In fact, switch() statements return NULL if there is no match, which could potentially cause bugs downstream. However, the current state of the code is fine because in the function that it is called, the argument passed to it is matched at the top, and errors there.

• What is the new behaviour (if this is a feature change)?

The functions will now error when there is no match instead of returning NULL, which could cause bugs.

• Does this PR introduce a breaking change? (What changes might users need to make in their application due to this PR?)

No.

• Other information:

• This PR closes #200.

• This change is probably unnecessary because the arguments passed to these helpers are checked and would error before being passed down to this function.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (00d337d) 98.90% compared to head (9881e08) 98.90%.

:exclamation: Current head 9881e08 differs from pull request most recent head 06ced1b. Consider uploading reports for the commit 06ced1b to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #205 +/- ## ======================================= Coverage 98.90% 98.90% ======================================= Files 8 8 Lines 548 550 +2 ======================================= + Hits 542 544 +2 Misses 6 6 ```

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