Fix exponential mesh bug

[rtimms]

Description

Fixes a bug with Exponential1DSubMesh when the left edge is non-zero.

Fixes #4988

Type of change

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

Important checks:

Please confirm the following before marking the PR as ready for review:

• No style issues: nox -s pre-commit
• All tests pass: nox -s tests
• The documentation builds: nox -s doctests
• Code is commented for hard-to-understand areas
• Tests added that prove fix is effective or that feature works

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 98.62%. Comparing base (eb9f5fc) to head (071ffd8).
Report is 106 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop    #4989   +/-   ##
========================================
  Coverage    98.62%   98.62%           
========================================
  Files          304      304           
  Lines        23755    23756    +1     
========================================
+ Hits         23428    23429    +1     
  Misses         327      327           

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[MarcBerliner]

Looks good, thanks!