stfc
Create PSyIR kernel arguments from new metadata classes (closes #2078)
This PR creates a subclass of the new metadata_to_argument_rules.py base class being implemented in PR #2048. This new subclass will create LFRic PSyIR kernel arguments. This is similar to the existing partial implementation in kernel_interface.py but uses the new metadata and will be complete, so will replace this code. It will also replace the existing kern_stub_arg_list.py which uses the old style string arguments and the old style metadata and is used when generating kernel stub code. Once complete this new class can then be used to generate kernel stub code by replacing the existing implementation in gen_kernel_stub.py.
As mentioned above this PR depends on PR #2048 being on master before it can proceed.
There's quite a long way to go to get the tests to pass flake8 - they're very much a WIP with variables assigned to but never used.
Following what Rupert wrote in the description of this PR, I've modified gen_kernel_stub so that it uses the new functionality in this PR rather than the old, fparser1-based version. This should help me understand what this PR is doing.