Implement wcll option using inputted pressure drop within dcll model

[kj5248]

Description

Resolves #3305.

Split up previous DCLL thermo_hydraulic_model to function for both primary_pumping=2 and 3 (new). This alongside changing the pumppower function so that the adiabatic index is based upon whether it is helium or water rather than just assuming helium means a WCLL blanket configuration can be modeled.

For primary_pumping=3 to work 4 pressure drop variables were added to primary_pumping_variables:

• dp_fw_blkt -> pressure drop across first wall and blanket
• dp_fw -> pressure drop across first wall
• dp_blkt -> pressure drop across blanket
• dp_liq -> pressure drop across liquid metal breeder

These can be used as inputs with primary_pumping=3, iblanket=5, ipupm=0, icooldual=1, and coolwh=2 to get a WCLL blanket when running PROCESS.

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.

[chris-ashe]

Happy with the doc additions and the removal of now removed models

[ajpearcey]

This is ready for merging