Add the refactored NN back into SAM

[jatkinson1000]

Add the refactored NN for convection parameterisation back into SAM.

• Update the README so it is clear what this repo is and how it differs from the original
• Adapt SAM to use our implementation
• Switch between Janni and new with namelist flag
• Import our source code from Convection repo
• Check it compiles and runs OK
• Compare outputs (and document in README how this can be manually diff'd)

[dorchard]

Great. First two tasks look done? Can we do a submodule for task 3 (to import from the convection repo)?

[jatkinson1000]

Great. First two tasks look done? Can we do a submodule for task 3 (to import from the convection repo)?

Mostly, just need to check and then I'll tick off.
Considered a submodule, but just want the source files rather than the whole repo, and preferably adjacent to the code rather than in a subdirectory for building purposes... Can discuss later today perhaps?

[raehik]

Some conflicting function/subroutine names between the original and refactored NN code made the "switching" awkward. @dorchard suggested we gate with CPP instead -- looking into it next.