Added RGBA to RGB conversion in preprocessing

[sarthakpati]

Fixes #383

Proposed Changes

• added preprocessing code for RGBA to RGB conversion using PIL
• variable renamed in preprocessing submodule for clarity

Checklist

• I have read the CONTRIBUTING guide
• My PR is based from the current GaNDLF master
• Non-breaking change (would not break existing functionality): please provide as many details as possible for any breaking change
• Function/class source code documentation added/updated
• Code has been blacked for style consistency
• If applicable, version information has been updated in GANDLF/version.py
• If adding a git submodule, add to list of exceptions for black styling in pyproject.toml file
• Usage documentation has been updated, if appropriate
• History has been updated, if appropriate
• Tests added or modified to cover the changes; if coverage is reduced, please give explanation
• If customized dependency installation is required (i.e., a separate pip install step is needed for PR to be functional), please ensure it is reflected in all the files that control the CI, namely: python-test.yml, and all docker files [1,2,3,4]

[dagshub]

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[codecov]

Codecov Report

Merging #384 (7c84827) into master (a4a9ce8) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #384      +/-   ##
==========================================
+ Coverage   91.68%   91.73%   +0.04%     
==========================================
  Files          93       94       +1     
  Lines        5450     5479      +29     
==========================================
+ Hits         4997     5026      +29     
  Misses        453      453              

Flag	Coverage Δ
unittests	91.73% <100.00%> (+0.04%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
GANDLF/data/preprocessing/__init__.py	100.00% <100.00%> (ø)
GANDLF/data/preprocessing/rgba2rgb.py	100.00% <100.00%> (ø)
testing/test_full.py	98.27% <100.00%> (+<0.01%)	⬆️

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