TCI: Fix `minmod` (!178)

The formulae isn't complicated, but I still messed up doing so.

Fixing it makes TOV evolutions in symmetric domains now remain symmetric, kind of.

Because `minmod` is used with almost all slope limiters, I had to update quite a few reftest too, although they overall quality of the results did not change much.

I wondering whether this fix makes now the WENO version of DG also work, will check in a separate PR.