Add GUI, XLSX support, and documentation

[timholy]

This adds an interactive GUI, creatively called gui, for analyzing collections of images. While the automated segmentation works so well it could probably be entirely automated, I felt uncomfortable with the idea of not having any "eyes" on the results.

Altogether this adds:

• the GUI
• support for writing *.xlsx files for those who want to analyze the resulting data with Excel. JLD2 files are also saved for later use from Julia.
• support for globs for pattern-matching input files
• user documentation

[codecov]

Codecov Report

Attention: Patch coverage is 98.71795% with 2 lines in your changes missing coverage. Please review.

Project coverage is 99.06%. Comparing base (eec4d64) to head (305d75a).
Report is 7 commits behind head on main.

Files with missing lines	Patch %	Lines
src/gui.jl	99.10%	1 Missing ⚠️
src/xlxs.jl	97.67%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##              main       #5      +/-   ##
===========================================
- Coverage   100.00%   99.06%   -0.94%     
===========================================
  Files            4        4              
  Lines           80      214     +134     
===========================================
+ Hits            80      212     +132     
- Misses           0        2       +2     

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