In order to make multirate work, we need to introduce new registration routines that explicitly register variables with the "slow" sector, i.e. those variables which are integrated by the lower order scheme. This also means that we require new "accumulator" parameters indicating how many "slow" variables we want to register.
Flags indicate whether it is time to execute slow RHS computation. For instance, in the RK4-RK2 scheme, there are 4 substeps in total, but the RK2 RHS are only evaluated in the very first and in the very last step of the four substeps.