rename i and j in power transformer transform & inverse transform

[veni-vidi-vici-dormivi]

Renamed i and j indexes to gridcell and year for better readability in PowerTransformerVariableLambda.transform() and inverse_transform.

[veni-vidi-vici-dormivi]

Sorry about yet another PR but I had to do this before I forget about it. Should be easy to merge though 🤞

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes are missing coverage. Please review.

Project coverage is 81.94%. Comparing base (9b0b76b) to head (2e77ec0). Report is 33 commits behind head on main.

:exclamation: Current head 2e77ec0 differs from pull request most recent head 3ea81a4. Consider uploading reports for the commit 3ea81a4 to get more accurate results

Files	Patch %	Lines
mesmer/mesmer_m/power_transformer.py	0.00%	7 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #444 +/- ## ========================================== - Coverage 87.90% 81.94% -5.96% ========================================== Files 40 44 +4 Lines 1745 1939 +194 ========================================== + Hits 1534 1589 +55 - Misses 211 350 +139 ``` | [Flag](https://app.codecov.io/gh/MESMER-group/mesmer/pull/444/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=MESMER-group) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/MESMER-group/mesmer/pull/444/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=MESMER-group) | `81.94% <0.00%> (-5.96%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=MESMER-group#carryforward-flags-in-the-pull-request-comment) to find out more.

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