Better treatment of applying the right rate-modifier function when
determining the maximal rate in Multi.hpp

We now check whether on of the two particles is static, and in that
case take the modified rate of the mobile particle.

This however does not resolve the principal problem yet, which is
that we do not acquire enough information about the actual situation
in this procedure.
Treat case D_2D=0 in MixedPair2D3D

We now get these cases whenever a 3D particle attempts to form a
mixed pair with a static cylinder-plane interface particle, which
nominally is a plane/2D particle.
Forgot to save and re-load decay reaction rules in load/save system

Now it works.