Fairmat 2024: proposal on electron microscopy (EM)

[lukaspie]

• Refactored version for NXem including feedback from the EM community
• Refactoring of NXimage, NXspectrum_set base classes and examples for their usage
• Examples for documenting method-specific results with some contextualization for processing EBSD as an example for diffraction, EDS for spectroscopy, as well as images
• More complete representation of microscope components
• Connection to all concepts within EMglossary community-agreed upon terminology set where it fits
• Backed by example data converted to NeXus files using NXem for at least one example for each microscope manufacturer and thousands of cases for EBSD as a specific technique

This PR depends heavily on others that make changes to existing base classes or introduce base classes that are to be used outside of APM as well. Thus, it will be much smaller once the discussion on these PRs is done and this particular branch here has been rebased:
#1407
#1408
#1410
#1412
#1413
#1414
#1415
#1419
#1420 
#1486

There are however several new base classes that are currently only used within NXapm that are introduced here:

• NXaberration
• NXcg_ellipsoid_set
• NXcg_face_list_data_structure
• NXcg_geodesic_mesh
• NXcg_half_edge_data_structure
• NXcg_primitive_set
• NXcg_triangle_set
• NXcg_triangulated_surface_mesh
• NXcg_unit_normal_set
• NXcorrector_cs
• NXcrystal_structure
• NXebeam_column
• NXem_correlation
• NXem_ebsd
• NXem_eds
• NXem_eels
• NXem_img
• NXem_method
• NXem_msr
• NXem_sim
• NXevent_data_em
• NXevent_data_em_set
• NXibeam_column
• NXinteraction_vol_em
• NXoptical_system_em
• NXscanbox_em
• NXspectrum_set

For a full list of dependant changes, see #1465