Start transfer for all tomo multigrid slots with sessions

[stephen-riggs]

Previously we only started tomo multigrid transfer when an mdoc appears. This changes it to start when a Session.dm appears so that all search maps get transferred, but meaning that empty cassette slots do not create a raw.

I also put a name on the analysers as this will help with debugging when we come to attempt the full tomo multigrid changes described in #792

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 51.43%. Comparing base (84e0b4a) to head (5dff919).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #793   +/-   ##
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  Coverage   51.43%   51.43%           
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  Files          96       96           
  Lines       10101    10101           
  Branches     1322     1322           
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  Hits         5195     5195           
  Misses       4634     4634           
  Partials      272      272           

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