Add handling for CL metadata in DM reader

[jlaehne]

Description of the change

Added import of various CL specific metadata fields to DM reader

Progress of the PR

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

[jlaehne]

Note that #2095 touches this PR. For the moment, I have not implemented microscope specific metadata under an SEM node, but simply set the acquisition_mode parameter to SEM instead of STEM/TEM under the TEM node. Current metadata structure would require a separate node, but I think the proposal from #2095 would make sense and would result in the metadata going one level up or the TEM being renamed to a more general EM to separate it from detector specific information under Acquisition_instrument.

[jlaehne]

How do I get a codecov for this PR?

[ericpre]

codecov comes from a build on travis and I am afraid that it is not working well at the moment. Until yesterday, there was a failure with latest h5py version and as of this morning there is still a failure introduced in #2437.
On top of that, travis significantly reduced their capacity of their free tier offer recently and there is now a massive queue: https://www.traviscistatus.com

[ericpre]

@jlaehne, the coverage is working now!

[codecov]

Codecov Report

Merging #2590 (4438e47) into RELEASE_next_minor (906da79) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@                  Coverage Diff                   @@
##           RELEASE_next_minor    #2590      +/-   ##
======================================================
+ Coverage               76.44%   76.46%   +0.01%     
======================================================
  Files                     202      202              
  Lines                   29734    29741       +7     
  Branches                 6490     6494       +4     
======================================================
+ Hits                    22729    22740      +11     
+ Misses                   5205     5203       -2     
+ Partials                 1800     1798       -2     

Impacted Files	Coverage Δ
hyperspy/io_plugins/digital_micrograph.py	67.65% <100.00%> (+0.80%)	⬆️
hyperspy/signal.py	75.61% <0.00%> (+0.09%)	⬆️

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[jlaehne]

Mac build was aborted, did not fail.