Add option to call PC2 erosion with CASIM

### Are there any linked Issues or Pull Requests?

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### Brief description

By default, PC2 erosion is done in the call to "pc2_hom_conv" alongside the convection call. In the UM, if not using a convection scheme, the erosion could be done in a call to "pc2_turbulence_ctl" alongside the Wilson-Ballard microphysics call. But if using CASIM instead of Wilson-Ballard, Pc2 erosion would be skipped entirely.

We need to check; in LFRic is the erosion done anywhere if not using convection? If not, we probably want to add a new optional PC2 erosion call in slow physics that is not explicitly tied to either Wilson-Ballard or CASIM?

### Further details of the issue.

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