Simplify axes conversion code

[jlaehne]

Description of the change

Addresses point 1 in #152. The axis conversion code was hard to read as routines were repeated for different situations, therefore

• reduced the differences between inplace=True/False cases
• and created a generalized utility function to combine code that is universal to the conversions and then call the correct subfunctions

Paves the way for more generalized unit conversion handling (i.e. any conversion between nm, eV and cm^-1).

Progress of the PR

• [x] Change implemented (can be split into several points),
• [x] docstring updated (if appropriate),
• [n/a] update user guide (if appropriate),
• [x] added tests,
• [ ] added line to CHANGELOG.md,
• [x] ready for review.

[codecov-commenter]

Codecov Report

Base: 100.00% // Head: 100.00% // No change to project coverage :thumbsup:

Coverage data is based on head (6aa40f2) compared to base (8367838). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #164 +/- ## ========================================= Coverage 100.00% 100.00% ========================================= Files 11 11 Lines 548 503 -45 ========================================= - Hits 548 503 -45 ``` | [Impacted Files](https://codecov.io/gh/LumiSpy/lumispy/pull/164?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LumiSpy) | Coverage Δ | | |---|---|---| | [lumispy/signals/luminescence\_spectrum.py](https://codecov.io/gh/LumiSpy/lumispy/pull/164/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LumiSpy#diff-bHVtaXNweS9zaWduYWxzL2x1bWluZXNjZW5jZV9zcGVjdHJ1bS5weQ==) | `100.00% <100.00%> (ø)` | | | [lumispy/utils/axes.py](https://codecov.io/gh/LumiSpy/lumispy/pull/164/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LumiSpy#diff-bHVtaXNweS91dGlscy9heGVzLnB5) | `100.00% <100.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LumiSpy). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=LumiSpy)

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