Added casths_fraction and casths as iteration variables

[chris-ashe]

Description

casths_fraction and casths have now been implemented as iteration variables

Checklist

I confirm that I have completed the following checks:

• 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.

[mkovari]

It would be nice at some point to add the relevant reference dimension (total coil thickness at the inner radius of the WP) to Figure 1 of the docs.

Or just the equation
total coil thickness at the inner radius of the WP = wwp1 + 2 * casths

[mkovari]

You could doo with a check to make sure that the two new iteration variables are not used at the same time.

[chris-ashe]

You could doo with a check to make sure that the two new iteration variables are not used at the same time.

I have checked and with both variables iterating there is no difference to the number of steps to convergence, as default PROCESS will still override casths if casths_fraction is defined in the input. This is explained in the docs for the user

[timothy-nunn]

Implementation correct. Others seem happy with the change also.