[Improvement] Include symmetry preserving summations

[jaykalinani]

Implement symmetry preserving summations which take care of the associative property. See this paper for details: https://www.sciencedirect.com/science/article/pii/S0045793018308399

[rhaas80]

Note: for this to work you need to apply the same summation in a deterministic order for all operations, including all of CarpetX's prolongation operations (and all derivatives in Z4c, which could eg be done inside out instead of left-to-right for central derivatives and in the direction of beta for the advection ones). 

This exists an unpublished branch of GRhydro that does that and a (public but not advertised) branch of Carpet: rhaas/SymmetricOperators in case you want to peek (similar branches exist for other thorns as well).

Does work indeed (I don't think there is a paper that explicitly shows the difference but I have a presentation on it that I used for a job interview once), but is lots of work and fragile of course (and unphysical since nature is not that symmetric) and one should include a controllable physical non-symmetry in the ID, really).