incremental psyclone

Todo: Needs definition. What are the things which, when changed, must trigger reprocessing of a file?

- Obviously the preprocessed x90 is one.
- There can be multiple "kernel root" folders too. Should we rebuild if _anything_ in those changes, or can/should we parse and discover just the files?
- Anything else?

Something's been updated in LFRic since we pinned the version, too. I believe it now uses big X90s and little x90s to differentiate preprocessed source.

_"...the basic rule is that PSyclone needs to be re-run if the algorithm (.x90) file, the kernels (included by the .x90) or the transformation script change."_

_"The transformation script is passed to PSyclone as an argument. That script is a python script..."_

No files, tests, or entry points are named. Start by defining the complete set of inputs that must trigger PSyclone reprocessing, including preprocessed X90 files, kernel roots, and the Python transformation script. Done means the incremental rebuild rules are agreed and account for the LFRic naming distinction.

Written by the indexing model from the issue text.