Refactor tools for ssx and I19

[noemifrisina]

Closes #115
Closes #55
Part of #57

• Refactor SSX using new tools
• Refactor I19 writer using new tools
• Remove obsolete writers
• Add a couple of tests
• Tidy up

[codecov]

Codecov Report

Merging #123 (d743741) into main (02f495a) will increase coverage by 2.76%.
The diff coverage is 39.36%.

❗ Current head d743741 differs from pull request most recent head 5bffd61. Consider uploading reports for the commit 5bffd61 to get more accurate results

@@            Coverage Diff             @@
##             main     #123      +/-   ##
==========================================
+ Coverage   34.17%   36.94%   +2.76%     
==========================================
  Files          42       43       +1     
  Lines        4041     4028      -13     
  Branches      826      796      -30     
==========================================
+ Hits         1381     1488     +107     
+ Misses       2529     2408     -121     
- Partials      131      132       +1     

Impacted Files	Coverage Δ
src/nexgen/beamlines/I19_2_gda_nxs.py	0.00% <0.00%> (ø)
src/nexgen/beamlines/I19_2_nxs.py	0.00% <0.00%> (ø)
src/nexgen/beamlines/SSX_Eiger_nxs.py	0.00% <0.00%> (ø)
src/nexgen/beamlines/SSX_Tristan_nxs.py	0.00% <0.00%> (ø)
src/nexgen/command_line/ED_nexus.py	0.00% <0.00%> (ø)
src/nexgen/command_line/cli_utils.py	0.00% <0.00%> (ø)
src/nexgen/command_line/nexus_generator.py	0.00% <0.00%> (ø)
src/nexgen/nxs_write/NXmxWriter.py	0.00% <0.00%> (ø)
src/nexgen/nxs_copy/__init__.py	34.16% <23.52%> (+0.25%)	⬆️
src/nexgen/nxs_write/EDNexusWriter.py	34.09% <33.33%> (-2.28%)	⬇️
... and 15 more

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