Upstream `open_operators` from Decapodes to DiagX

Currently, functionality that expands definitions of differential operators (i.e. `open_operators`) is used during the `gensim` pipeline in Decapodes.

Since this functionality only changes the Decapode model itself, and there is no surrounding state edited by this functionality, this can be upstreamed to this package.

Note that similar functionality is also being hit by PR #37 , which "alias"es operators, effectively doing the same thing.

Since we are effectively replacing some of the imperative opening code with a more-or-less declarative interface, this migration should ensure all relevant definitions and tests are copied over, but not necessarily any function definitions outright.