Dissallow use of multiple reduction operators for the same field across all particles.

[LonelyCat124]

I.e. don't allow

    part1 += a
    part2 -= a

It might be ok, however I'm not sure and its easier to disallow this during compilation and just make them use += for both.

[LonelyCat124]

I think in principle this is not an issue at all. My understanding is that all that would happen in this case is that there would exist reduction instances for + and -, and these would be handled separately. The issue right now is that the custom mapper would not be able to understand this, and would likely hit assert(false).