Fix MSA signaltype

[jlaehne]

Description of the change

As brought up in #37, the current implementation of the MSA writer does not comply to the format definitions according to
https://the-mas.org/wp-content/uploads/2018/11/emmff_ascii.txt

In particular, HyperSpy signal_types where written to MSA 1:1, even if they deviate from the allowed strings according to the format definition.

This PR correctly converts HyperSpy signal strings to the corresponding MSA ones when writing. On the reading side, the resulting signal_types don't necessarily correspond to the HyperSpy ones. Correct conversion to signal types of HypersSpy has to be done on the HyperSpy/LumiSpy side (e.g. choosing which voltage corresponds to SEM or TEM) as proposed in #37.

Progress of the PR

• Change implemented (can be split into several points),
• [n/a] update docstring (if appropriate),
• [n/a] update user guide (if appropriate),
• add an changelog entry in the upcoming_changes folder (see upcoming_changes/README.rst),
• Check formatting changelog entry in the readthedocs doc build of this PR (link in github checks)
• add tests,
• ready for review.

Minimal example of the bug fix or the new feature

import hyperspy.api as hs
s = hs.load("rosettasciio/rsciio/tests/msa_files/minimum_metadata.msa")
s.metadata.Signal.signal_type = "EDS_SEM"
s.save("tmp.msa")
s2 = hs.load("tmp.msa")

Note that this example can be useful to update the user guide.

[codecov]

Codecov Report

Base: 82.73% // Head: 82.90% // Increases project coverage by +0.17% 🎉

Coverage data is based on head (440ce8d) compared to base (57fccc4).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head 440ce8d differs from pull request most recent head 162a12d. Consider uploading reports for the commit 162a12d to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #39      +/-   ##
==========================================
+ Coverage   82.73%   82.90%   +0.17%     
==========================================
  Files          40       40              
  Lines        8032     8042      +10     
  Branches     1857     1860       +3     
==========================================
+ Hits         6645     6667      +22     
+ Misses        920      911       -9     
+ Partials      467      464       -3     

Impacted Files	Coverage Δ
rsciio/msa/api.py	80.23% <100.00%> (+8.62%)	⬆️

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[ericpre]

Should it have a changelog entry?