Replace stellarator only variable for first wall coolant channel with general use variable

[kj5248]

Closes #3109

Description

In stellarator there were variables afwi and afwo for the coolant inner channel radius for the FW inboard and outboard respectively. These variables seemed to have the same value in all the stellarator files and is comparable to the tokamak variable radius_fw_channel which is the inner channel radius for the FW (inboard and outboard).

For this reason, and to also allow for the potential of using the dcll blanket model with the stellarator, these variables were replaced with the tokamak variable radius_fw_channel.

Also had to make a change to the stellarator regression file as the default value for afwi and afwo was 0.008 rather than the default of radius_fw_channel which is 0.006.

Closes #3109

Checklist

I confirm that I have completed the following checks:

• My changes follow the PROCESS style guide
• I have justified any large differences in the regression tests caused by this pull request in the comments.
• I have added new tests where appropriate for the changes I have made.
• If I have had to change any existing unit or integration tests, I have justified this change in the pull request comments.
• If I have made documentation changes, I have checked they render correctly.
• I have added documentation for my change, if appropriate.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 31.74%. Comparing base (204b622) to head (31a5277).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3580   +/-   ##
=======================================
  Coverage   31.74%   31.74%           
=======================================
  Files          85       85           
  Lines       20161    20161           
=======================================
  Hits         6400     6400           
  Misses      13761    13761           

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[timothy-nunn]

Rebase onto main. Are we expecting the value changes in the regression test if this a variable rename (could be fixed by rebasing onto main).

[timothy-nunn]

Regression failures (except Stellarator files) are caused by #3602

Stellarator regression test differences are being caused by what I suspect to be a bug in the stellarator. radius_fw_channel was being used in some places in the Stellarator before this change:

PROCESS/process/stellarator.py

Lines 347 to 349 in 204b622

	build_variables.dr_fw_inboard = (
	2.0e0 * fwbs_variables.radius_fw_channel + 2.0e0 * fwbs_variables.dr_fw_wall
	)

However, radius_fw_channel was at its default value of 0.006. Following this change, radius_fw_channel is 0.008 at this point in the code. Testing by replacing this use of radius_fw_channel with 0.006 causes the regression tests to pass.

I believe that this changes the code to be correct and the use of 0.006 before was an error.