Bruker XRF fix

[ericpre]

Fix #323.

Progress of the PR

• Don't raise error when the beam energy can't be found in bcf xrf files,
• Add export_xml utility function,
• update docstring (if appropriate),
• update user guide (if appropriate),
• add a changelog entry in the upcoming_changes folder (see upcoming_changes/README.rst),
• Check formatting of the changelog entry (and eventual user guide changes) in the docs/readthedocs.org:rosettasciio build of this PR (link in github checks)
• add tests,
• ready for review.

Minimal example of the bug fix or the new feature

from rsciio.bruker import export_xml
export_xml("your_msa_file.msa")

[codecov]

Codecov Report

Attention: Patch coverage is 77.41935% with 7 lines in your changes missing coverage. Please review.

Project coverage is 87.81%. Comparing base (88923bf) to head (7926c57).

Files with missing lines	Patch %	Lines
rsciio/bruker/_api.py	63.15%	1 Missing and 6 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #326      +/-   ##
==========================================
- Coverage   87.84%   87.81%   -0.03%     
==========================================
  Files          85       86       +1     
  Lines       11180    11198      +18     
  Branches     2280     2284       +4     
==========================================
+ Hits         9821     9834      +13     
- Misses        860      861       +1     
- Partials      499      503       +4     

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[ericpre]

Waiting for a smaller file from #323 (comment).

[sem-geologist]

Generally that Tornado instrument looks to me "Fascinating" as header clearly contains absolutely no information about primary energy of the beam.