Axis conversion for TransientSpec

[jlaehne]

Description of the change

Move axis conversion code to CommonLuminescence class to make it available to LuminescenceTransientSpectrum signals.

Handle inheritance for slicing of LuminescenceTransientSpectrum:

• Create 1D version of signal_type = 'TransientSpec' (class TransientSpectrumCasting)
• If TransientSpec is initialized in 1D with axes_manager[-1].units being a time unit, switch to LumiTransient class, otherwise switch to Luminescence class

Progress of the PR

• [x] move axis conversion code,
• [x] Implement workaround for https://github.com/hyperspy/hyperspy/issues/3294
• [x] Handle slicing of TransientSpec,
• [ ] add section to user guide,
• [x] add tests for TransientSpec case,
• [ ] added line to CHANGELOG.rst,
• [ ] ready for review.

[codecov-commenter]

:warning: Please install the to ensure uploads and comments are reliably processed by Codecov.

Codecov Report

Attention: Patch coverage is 97.95918% with 2 lines in your changes missing coverage. Please review.

Project coverage is 99.46%. Comparing base (9c402c0) to head (88d1c3d). Report is 20 commits behind head on main.

Files with missing lines	Patch %	Lines
lumispy/signals/common_luminescence.py	97.53%	2 Missing :warning:

:exclamation: Your organization needs to install the Codecov GitHub app to enable full functionality.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #205 +/- ## =========================================== - Coverage 100.00% 99.46% -0.54% =========================================== Files 12 12 Lines 556 566 +10 =========================================== + Hits 556 563 +7 - Misses 0 3 +3 ```

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