XIOS: allow a field to be read and written with different types

[joewallwork]

In nextSIM-DG, fields that are treated as DGFields by the model are often initialised from HField data in an initial condition file. They may subsequently be written out as DGField restarts and later runs can read in the restart from DGField. This means that we need to separate out the reading and writing part of a field to improve generality. The overloaded = operator of ModelArray may be used to "project" from HField to DGField so we'll probably need to make use of this.

[joewallwork]

XIOS error from the June '23 config illustrating the issue:

In file "model_to_client_source_filter.cpp", function "void xios::CModelToClientSourceFilter::streamData(xios::CDate, const xios::CArray<double, N>&) [with int N = 3]",  line 34 -> [ Awaiting data of size = 4503, Received data size = 13509 ] The data array does not have the right size! grid =  --> HGrid

(1) **************** void cxios_write_data_k83_hdl(xios::CField*, double*, int, int, int)