contrast GUI: convert new minimum and maximum values to float

[jwahlstrand]

Implements suggestion from @kimikage

Fixes #310

I verified that this still avoids the crash in #308

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 2 lines in your changes missing coverage. Please review.

Project coverage is 85.74%. Comparing base (27fb192) to head (6f632bd).

❗ Current head 6f632bd differs from pull request most recent head ca4de2b

Please upload reports for the commit ca4de2b to get more accurate results.

Files	Patch %	Lines
src/contrast_gui.jl	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #311      +/-   ##
==========================================
+ Coverage   85.01%   85.74%   +0.73%     
==========================================
  Files           6        6              
  Lines         814      870      +56     
==========================================
+ Hits          692      746      +54     
- Misses        122      124       +2     

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[kimikage]

I have once again checked the implementation of the contrast GUI and noticed several design issues.
However, I think it is a good idea to release this first as a quick fix.

[jwahlstrand]

Thanks. The code does somehow seem a bit complicated for what it does. I'll merge this now and do another release.