Fix smart load | Make extension check case-insensitive

[qin-yu]

Adeleh from University of Potsdam sent me email about PlantSeg can't open her .JPG and .TIF files. This PR improves the extension checking throughout PlantSeg and makes smart_load()s consistent.

[codecov]

Codecov Report

Attention: Patch coverage is 54.54545% with 5 lines in your changes missing coverage. Please review.

Project coverage is 64.71%. Comparing base (c0a79c7) to head (63d38eb).
Report is 5 commits behind head on master.

Files with missing lines	Patch %	Lines
plantseg/io/io.py	60.00%	2 Missing ⚠️
plantseg/core/image.py	0.00%	1 Missing ⚠️
plantseg/io/h5.py	50.00%	1 Missing ⚠️
plantseg/viewer_napari/widgets/proofreading.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #368   +/-   ##
=======================================
  Coverage   64.71%   64.71%           
=======================================
  Files          71       71           
  Lines        5201     5201           
=======================================
  Hits         3366     3366           
  Misses       1835     1835           

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