Remove unused code and refactor

[adamltyson]

Follows on from #74 & #76. This PR:

• Moves some code to brainglobe-utils (requires https://github.com/brainglobe/brainglobe-utils/pull/25 to be merged & released before the tests will pass)
• Removes some unused code
• Slightly simplifies directory structure

[codecov[bot]]

Codecov Report

Attention: 20 lines in your changes are missing coverage. Please review.

Comparison is base (2b19243) 85.08% compared to head (845b25d) 79.43%.

Files	Patch %	Lines
tests/brainmapper/test_unit/test_prep.py	52.63%	18 Missing :warning:
brainglobe_workflows/brainmapper/analyse.py	0.00%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #79 +/- ## ========================================== - Coverage 85.08% 79.43% -5.65% ========================================== Files 26 18 -8 Lines 1448 924 -524 ========================================== - Hits 1232 734 -498 + Misses 216 190 -26 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.