Remove unused code and refactor

[adamltyson]

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

• Moves some code to brainglobe-utils (requires Add misc functionality from brainmapper brainglobe-utils#25 to be merged & released before the tests will pass)
• Removes some unused code
• Slightly simplifies directory structure

[codecov]

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 ⚠️
brainglobe_workflows/brainmapper/analyse.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            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     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[sfmig]

LGTM - seems much lighter now ✨