implement abrupt HY/CG interface

[J-Hizzle]

In the latest TR version of AdResS, there is an abrupt switching off of interactions at the HY/CG interface, which is called the Delta/TR interface in this context. This requires the implementation of a new type of weighting function that is simply 1 throughout HY and 0 in CG. Whilst this would technically be already achieved in the current version of MRMD by setting the HY region diameter to 0, measurement of physical observables would be done up to the Delta/TR interface, which would be stained by the interface artifacts and thus this needs to be circumvented.

I suggest refactoring Slab.hpp:96-109 as a dedicated function and adding another function that would make the interface abrupt.

[XzzX]

Why exactly is setting HY diameter to 0 a problem? I am open for some refactoring.

[J-Hizzle]

One Example would be that the thermodynamic force is only applied within the HY region, if I understand ThermodynamicForce.cpp:147 correctly. In the case of a HY region with 0 diameter, this would break the thermodynamic force as it is at the moment. I could also imagine there being other cases like that within MRMD when setting the HY region to 0 diameter.

Short notice on the application region of the thermodynamic force: I will most probably need to rework this as well, since for the abrupt interface, the thermodynamic force needs to be applied also within the CG region.

[XzzX]

fixed by #31