Add functionality to handle subcycling #55
Description
In precice/tutorials#666 the negotiation of the time step (dt) value with preCICE is implemented in the two-scale heat conduction tutorial participant macro-dumux. This negotiation can be handled inside the adapter, so that it is completely hidden from the user. The macro-dumux is already quite general, so the implementation can pretty much be directly ported to the adapter by modifying the existing functions.