New Operator

An operator needs to implement the following things:

• bc_width for boundary handling

• couldSafeEval(e, i) check if the operator can handle calculation of e at i

• eval_safe(e, i) evaluate the expression e at i with all checking enabled

• eval(e, i) evaluate the expression e at i with only the critical path

• prepare(e) prepare all meta infos of expression e. Usually include name, mesh, bc & ranges.

• ResultType<Op, T...> defining the result of the operation, containing a type type for the result full type and a core_type for Expression<...> wrapped core type

• internal::ExprTrait<Expression<Op, T...>> defining all the meta infos of the intermediate expression. Should include same meta infos as an ExprTrait for a concrete field of that type