Update cryostat docs

[chris-ashe]

Description

This pull request includes several changes to the naming conventions for cryostat-related variables across multiple files, ensuring consistency and clarity in the codebase. The most important changes involve renaming variables and updating documentation to reflect these changes.

🔄 Namespace changes

Variables

• rdewex -> r_cryostat_inboard
• rpf2dewar -> dr_pf_cryostat
• hcryopf -> dz_pf_cryostat
• zdewex -> dz_cryostat_half_inside
• vdewex -> vol_cryostat
• ddwex -> dr_cryostat
• clhsf -> f_z_cryostat
• clh1 -> dz_tf_cryostat

🐛 Bugs

• The previous calculation for the bolume of the cryostat structure was:

fwbs_variables.vdewex = (
            (2.0 * np.pi * fwbs_variables.rdewex) * 2.0 * fwbs_variables.zdewex
            + (2.0 * np.pi * fwbs_variables.rdewex**2)
        ) * build_variables.ddwex

Which wasnt easy to interpret so a new model for the cryostat structure volume has been introduced which is simply
dividing the cryostat into 2 nested cylinders and taking the internal volume from the bigger one. This caused a 1% change in the calculated volume compared to the tests (810 -> 818 $m^3$)

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

Attention: Patch coverage is 46.42857% with 15 lines in your changes missing coverage. Please review.

Project coverage is 30.32%. Comparing base (f208c63) to head (327bb37).
Report is 24 commits behind head on main.

Files with missing lines	Patch %	Lines
process/io/plot_proc.py	0.00%	5 Missing ⚠️
process/build.py	0.00%	4 Missing ⚠️
process/stellarator.py	0.00%	4 Missing ⚠️
process/geometry/cryostat_geometry.py	0.00%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3501   +/-   ##
=======================================
  Coverage   30.31%   30.32%           
=======================================
  Files          80       80           
  Lines       19534    19537    +3     
=======================================
+ Hits         5922     5924    +2     
- Misses      13612    13613    +1     

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