PSyAD: incorrect initialisation of multi-data fields in test harness

[arporter]

Currently PSyAD does not generate correct initialise calls for multi-data fields in the created test harness since it has no way of knowing whether a field is multi-data. #868 is adding support for custom function spaces in LFRic and then it may be possible for PSyAD to determine whether a field is multi-data @TeranIvy?

Initialisation of a multi-data field (with 4 values at each dof) looks like:

multidata_fs => &
  function_space_collection%get_fs( mesh, element_order, Wtheta, 4, ndata_first=.false. )

call recon % initialise( vector_space = multidata_fs, &
     name = 'recon' , halo_depth = get_required_stencil_depth() )