CoMorph microphysics changes for CASIM 2nd ice category

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

_No response_

### Brief description

CoMorph has so-far only been run with a single ice-cloud prognostic, whereas CASIM uses 2 ice-cloud fields, one for small ice crystals and one for larger "snow". While CoMorph already includes "plumbing" to allow it to transport a 2nd ice category, it's in-plume microphysics scheme's implementation for this is incomplete:
a) It doesn't contain any processes to create "snow" within the updraft. We need to add a process which autoconverts "ice" to "snow". We can then tune this process to give a realistic partitioning of detrained ice between the two categories, which is important for CASIM and the radiative impact of the ice.
b) Collisions between snow and rain ought to produce graupel but at present these collisions only add mass onto the snow.
c) We might also want to allow collisions between snow and ice-cloud to increase snow-mass via "collection".

This will build upon some of the relevant changes already coded in this branch:
https://github.com/MetOffice/lfric_apps/commit/cc8671059e549b1f8da41404255feed6aeccfa81

### Further details of the issue.

_No response_