Introduce _substitute_pixel to simplify everything

[Saransh-cpp]

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

Fixes # (issue)

Type of change

Please add a line in the relevant section of CHANGELOG.md to document the change (include PR #) - note reverse order of PR #s.

• New feature (non-breaking change which adds functionality)
• Optimization (back-end change that speeds up the code)
• Bug fix (non-breaking change which fixes an issue)

Key checklist:

• Package installs: julia> ]dev .
• All tests pass: julia> ]test ChaoticEncryption
• The documentation builds: $ cd docs and then $ julia make.jl

Further checks:

• Code is commented, particularly in hard-to-understand areas
• Tests added that prove fix is effective or that feature works

Acknowledgements

This Pull Request template was taken from the excellent PyBaMM repository.

[github-actions]

Once the build has completed, you can preview any updated documentation at this URL: https://saransh-cpp.github.io/ChaoticEncryption.jl/previews/PR78/ in ~5 minutes

[codecov]

Codecov Report

Merging #78 (248f0d5) into master (8f6311c) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff            @@
##            master       #78   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            4         3    -1     
  Lines           60        51    -9     
=========================================
- Hits            60        51    -9     

Impacted Files	Coverage Δ
src/substitution.jl	100.00% <100.00%> (ø)

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