Further instrument server fixes + improving test coverage

[tieneupin]

• Corrected the logic for deciding on the rsync URL when uploading gain reference. It should look for the rsync URL key first before defaulting to the server URL.
• Leave None as-is in JSON data when preparing it in _start_dc()
• Captured the outputs of the post requests to the register_tomo_preproc_params() and register_tomo_proc_params() endpoints
• Added test for the upload_gain_reference() instrument server endpoint and for _get_murfey_url()

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 3 lines in your changes missing coverage. Please review.

Project coverage is 30.78%. Comparing base (2c595ca) to head (cc6477e).
Report is 3 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #570      +/-   ##
==========================================
+ Coverage   28.75%   30.78%   +2.02%     
==========================================
  Files          80       80              
  Lines       10621    10694      +73     
  Branches     1417     1431      +14     
==========================================
+ Hits         3054     3292     +238     
+ Misses       7465     7295     -170     
- Partials      102      107       +5     

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