[LFRic] User-defined DoF kernels should work with field vectors

[TeranIvy]

Support for user-defined kernels that operate on DoFs is available in PSyclone 3.0.0. However, there is a limitation on them to not accept field vectors. That is an issues as a lot of PSyKAl-lite code that refers to this functionality uses coordinate fields which are field vectors.

Historically, this limitation may have arosen from inability of PSyclone built-ins to deal with field vectors, but it is not required for user-defined kernels in LFRic. This issue removes the limitation and adds the relevant code generation.