Improve data sharing between GridOperators in OneStepGridOperator

The GridOperator interface makes it possible for the two operators that are part of a OneStepGridOperator to share state, which avoids duplicate calls to bind() etc. and duplication of data structures.

The default grid operator should make use of this facility. You can take a look at the MultiDomain grid operator for inspiration (e.g. the residual assembler engine at https://github.com/smuething/dune-multidomain/blob/master/dune/pdelab/multidomain/residualassemblerengine.hh contains a special data structure at the beginning of the file, which is shared among the residual engines of each operator using a shared_ptr).