Fixed `sliceDir` index

[eytanadler]

Purpose

In the addArbitrarySlices function, the slice direction was being indexed with the variable used to track the total slices so far, rather than the correct index looping over the slices to add.

Expected time until merged

Type of change

• [x] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (non-backwards-compatible fix or feature)
• [ ] Code style update (formatting, renaming)
• [ ] Refactoring (no functional changes, no API changes)
• [ ] Documentation update
• [ ] Maintenance update
• [ ] Other (please describe)

Testing

Checklist

• [ ] I have run flake8 and black to make sure the Python code adheres to PEP-8 and is consistently formatted
• [ ] I have formatted the Fortran code with fprettify or C/C++ code with clang-format as applicable
• [ ] I have run unit and regression tests which pass locally with my changes
• [ ] I have added new tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 41.45%. Comparing base (5f233c2) to head (8abd1cd).

Files	Patch %	Lines
adflow/pyADflow.py	0.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #351 +/- ## ======================================= Coverage 41.45% 41.45% ======================================= Files 13 13 Lines 4069 4069 ======================================= Hits 1687 1687 Misses 2382 2382 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.