Support fem.Function for CouplingMesh read_fields

[Snapex2409]

Currently, when constructing a CouplingMesh obj the read_fields dict needs contain fem.Functionspace as values.

Would it be possible to also allow fem.Function objects as values, similar to the write_fields dict.
Given the newest support of direct reading into fem.Function buffers, this would make the resulting code more readable.

read_functions = {
    'a': fem.Function(V),
    'b': fem.Function(V)
}
read_fields = {
    'a': V,
    'b': V
}

# ... some other preCICE setup code

for name, func in read_fields.items(): precice.read_data("mesh", name, dt, func)

This could then be simplified by dropping read_fields and using read_function as the new read_fields.