Cleaned up the number of integrators and removed the direct pointers to
PD connections so that only one datastructure is used.
Added an extension to the Boost library for serialization of
std::unordered_map
Some restructuring of the code. Ordinary state based PD has been added.
In addition an unordered map of PD neighbours is added to each particle.
The map contains all information needed for calculations on the
neighbours except for their position.
Changed the input velocity from being dimensionless to [m/s], and
converting it to dimensionless form in the contructor.
Major update - lots of things fixed and some structura changes.
Reset integrator.
Removed a debugging test.