Clarify DPD and thermostat interaction

[RudolfWeeber]

To my understanding, setting non_bonded_inter[i,j].dpd will only actually apply the forces, if the DPD thermostat is enabled

There are two issues:

• potential for accidents
• for kT=0, it is not clear, why a "thermostat"should be needed.

The coupling exists, because non_bonded interactions do not have state, but the random number generator drawing the noise for kT>0 has. Hence, the noise generaiton is handled as thermostat.

We could

1. handle the case of kT separately
2. continue to always require the thermostat to be set, but at least throw in on_integration_start() that(), if a DPD interaction is set without one.
3. handle kT=0 separately and otherwise throw in on_integraoitn_start()