Remove TBR calculation

[timothy-nunn]

Remove the Tritium Breeding Ratio model from PROCESS entirely

@stuartmuldrew @chris-ashe

[mkovari]

What's this all about?

[stuartmuldrew]

i_blanket_type=3 uses the i_blanket_type=1 CCFE HCPB blanket model, but extends it to include a TBR approximation based on fitting to models developed by Jonathan Shimwell for EUROfuion-DEMO.

The fitting allows for only three fixed blanket thicknesses set using blanket_thickness, with options THIN, MEDIUM or THICK. This overwrites any blanket thicknesses given in the input.

Discussions with Jonathan Shimwell indicated that this model is outdated and unlikely to be accurate based on latest considerations. It is proposed we remove the i_blanket_type=3 TBR extension and blanket_thickness.

[mkovari]

Fair enough.