Missing compilation error when manipulating VectorFields

[EmilyBourne]

The result of a call to VectorField::operator() is a Vector object. However this object is created from copies of the elements of the vectors. This happens because the memory returned from VectorField is non-contiguous. One solution would be to use std::tuple in Tensor and allow the ElementType to be a reference. This was tried in #109 however std::tuple methods are not supported on GPU. For now we have to live with this restriction. This prevents us from doing things like:

void update_element(host_t<VectorField<double, IdxRangeXY, NDTag<X, Y>>> my_vec_field, DVector<X, Y> new_val, IdxXY pos) {
    my_vec_field(pos) = new_val;
}

However currently this code does not raise an error. It simply modifies the temporary Vector that was created, leaving my_vec_field unchanged. Instead VectorField::operator() should return a const object to ensure that an error is raised to avoid accidental bugs when writing code like this.