Add request-scoped processing config

[sevein]

This commit removes support for processingMCP.xml. Instead, users are
asked to submit the processing configuration via the API. The workflow
had a couple of now unused link types removed and added a new LinkChoice
type removing the need of workflow chains.

Fixes #76.

[codecov-io]

Codecov Report

Merging #85 into main will decrease coverage by 0.24%.
The diff coverage is 49.10%.

@@            Coverage Diff             @@
##             main      #85      +/-   ##
==========================================
- Coverage   46.41%   46.17%   -0.25%     
==========================================
  Files         108      106       -2     
  Lines        8014     7822     -192     
  Branches     1051     1030      -21     
==========================================
- Hits         3720     3612     -108     
+ Misses       4115     4035      -80     
+ Partials      179      175       -4     

Impacted Files	Coverage Δ
a3m/cli/client/__main__.py	0.00% <0.00%> (ø)
a3m/cli/client/wrapper.py	0.00% <ø> (ø)
a3m/client/clientScripts/a3m_download_transfer.py	0.00% <0.00%> (ø)
...lient/clientScripts/assign_uuids_to_directories.py	0.00% <0.00%> (ø)
a3m/client/clientScripts/compress_aip.py	0.00% <0.00%> (ø)
.../clientScripts/create_sip_from_transfer_objects.py	0.00% <ø> (ø)
a3m/client/clientScripts/identify_file_format.py	0.00% <0.00%> (ø)
a3m/client/clientScripts/verify_sip_compliance.py	0.00% <0.00%> (ø)
...client/clientScripts/verify_transfer_compliance.py	0.00% <0.00%> (ø)
a3m/executeOrRunSubProcess.py	42.02% <ø> (-2.90%)	⬇️
... and 16 more

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