Set up and use -save-source-list and addmodel for predicting at full channel resolution

[tjgalvin]

This pull request arose after discussions around predicting a source model at full spectral resolution of a measurement set.

To-date, the model that has been added to a MS has been at the spectral resolution set by the sub-band interval controlled via the -channels-out option of wsclean. In this sense the model that has been added to the MS is constant over each sub-band interval, which limits the usability of the model for things like continuum subtraction.

Here an attempt is made to predict the model at full resolution by taking the component list produced by -save-source-list, which details all components, their scales, location and constrained SED, and which is then used by addmodel to predict. addmodel is part of the calibrate suite produced by Andre Offringa.

In terms of functionality this PR adds:

• tracking of the source list file
• optionally use addmodel to create the MODEL_DATA between imaging rounds
• building blocks to perform continuum subtraction

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 72.64151% with 29 lines in your changes missing coverage. Please review.

Project coverage is 60.91%. Comparing base (da6db25) to head (899bc64).

Files with missing lines	Patch %	Lines
flint/prefect/common/ms.py	34.78%	15 Missing ⚠️
flint/calibrate/aocalibrate.py	80.00%	5 Missing ⚠️
flint/imager/wsclean.py	80.76%	5 Missing ⚠️
flint/prefect/flows/continuum_pipeline.py	20.00%	4 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##             main     #190      +/-   ##
==========================================
+ Coverage   60.71%   60.91%   +0.20%     
==========================================
  Files          33       34       +1     
  Lines        5771     5852      +81     
==========================================
+ Hits         3504     3565      +61     
- Misses       2267     2287      +20     

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