Improve coverage, fix issue with variance conversion

[jlaehne]

Description of the change

• Fix some of the missing coverage mentioned in #133.
• Fix issue with to_eV and to_invcm changing the variance of the original signal when the variance is a single int/float and inplace=False (turned up as inplace=True was not covered by the test despite it should have been).
• Remove remnants of old HyperSpy axes scheme in solve_grating_equation that turned up as a missing coverage for the line checking the axes type version.

Progress of the PR

• [x] Change implemented,
• [x] ready for review.

[codecov-commenter]

Codecov Report

Merging #134 (5539b96) into main (004a7dd) will increase coverage by 1.78%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #134      +/-   ##
==========================================
+ Coverage   96.58%   98.36%   +1.78%     
==========================================
  Files          11       11              
  Lines         556      550       -6     
==========================================
+ Hits          537      541       +4     
+ Misses         19        9      -10     

Impacted Files	Coverage Δ
lumispy/signals/luminescence_spectrum.py	94.94% <100.00%> (+2.16%)	:arrow_up:
lumispy/utils/axes.py	100.00% <100.00%> (+1.93%)	:arrow_up:
lumispy/signals/cl_spectrum.py	100.00% <0.00%> (+1.69%)	:arrow_up:
lumispy/signals/common_luminescence.py	100.00% <0.00%> (+3.03%)	:arrow_up:

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