Add density_map

[timholy]

This adds the final feature intended before initial registration.

[codecov]

Codecov Report

Attention: Patch coverage is 84.37500% with 5 lines in your changes missing coverage. Please review.

Project coverage is 97.54%. Comparing base (eea067b) to head (5e8d3e0).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/segment.jl	86.66%	4 Missing ⚠️
src/gui.jl	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main       #6      +/-   ##
==========================================
- Coverage   99.06%   97.54%   -1.53%     
==========================================
  Files           4        4              
  Lines         214      244      +30     
==========================================
+ Hits          212      238      +26     
- Misses          2        6       +4     

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