resizes output vector to better handle vectors

[andcarl1364]

enhances MeshFunction to better handle vector variables which in turn helps general field transfers work with vectors as well

resolves: #3714 see also: #26084, #26045

[roystgnr]

What does this do?

_new_resize gets initialized to 0, then I don't see it ever being used again.

vec_dim, n_vector_vars, and n_scalar_vars get calculated (somewhat confusingly, but that's an existing problem with the overlapping terminology for scalar-valued-functions vs discrete-per-system-scalars) ... and then I don't see them ever being used again?

[andcarl1364]

my file didn't save correctly, let me do it again