Transpose centroid output

[jlaehne]

Description of the change

I realized that for a map the new centroid function, similar to e.g. valuemax and indexmax in HyperSpy returns a BaseSignal with 2 navigation dimensions and 0D signal, which theoretically makes sense, but is inconvenient for plotting as it plots including the navigator, which makes little sense for 0D signals, and colormap cannot be changed.

In contrast, estimate_peak_width returns the transposed signal, thus Signal2D object without navigation dimensions, which is more convenient for plotting - and is okay in the sense that it is just an image of values for the different positions. The as_signal command for parameters from a fit also results in Signal2D. I therefore think, it is the more sensible output here.

We should consider making that more consistent also in HyperSpy (e.g. with the 2.0 release).

As I was at it, I also removed optional parameters from the title if they are not set.

Progress of the PR

• Change implemented (can be split into several points),
• docstring updated (if appropriate),
• update user guide (if appropriate),
• added tests,
• ready for review.

Minimal example of the bug fix or the new feature

from lumispy.signals.luminescence_spectrum import LumiSpectrum
s = LumiSpectrum([[[1, 2, 3, 4, 5]] * 3] *4)
com = s.centroid()
com

now results in

<Signal2D, title: Centroid map, dimensions: (|3, 4)>

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: -0.55 ⚠️

Comparison is base (d3cf6ac) 100.00% compared to head (014c93b) 99.45%.

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Additional details and impacted files

@@             Coverage Diff             @@
##              main     #184      +/-   ##
===========================================
- Coverage   100.00%   99.45%   -0.55%     
===========================================
  Files           12       12              
  Lines          545      554       +9     
===========================================
+ Hits           545      551       +6     
- Misses           0        3       +3     

Impacted Files	Coverage Δ
lumispy/signals/luminescence_spectrum.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes

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