Function numel_struct_field in mex_utils.f90 not working

New function (tensor_unpack_shape) has already been implemented for the same purpose in mex_utils.f90.

numel_struct_field fixed provisionally as follows (uncommented lines fix the issue):

    function numel_struct_field(struct_pr, fieldname, struct_idx) result(n)
        mwPointer, intent(in) :: struct_pr
        character(len=*), intent(in) :: fieldname
        mwIndex, intent(in), optional :: struct_idx
        !mwSize :: n  ! intent(out)
        mwPointer :: n

        mwPointer :: field_pr
        ! mwPointer :: mxGetDimensions
        ! mwSize :: mxGetNumberOfDimensions
        ! mwSize :: n_dims
        ! integer(kind=4), dimension(:), allocatable :: dims
        mwPointer :: mxGetNumberOfElements

        field_pr = get_field_pr(struct_pr, fieldname, struct_idx)
        ! n_dims = mxGetNumberOfDimensions(field_pr)
        ! allocate (dims(n_dims))
        ! call mxCopyPtrToInteger4(mxGetDimensions(field_pr), dims, n_dims)
        ! n = product(dims)
        ! deallocate (dims)
        n = mxGetNumberOfElements(field_pr)
    end function