Define mechanical interaction model

[bjpalmer]

Define a mechanical interaction model between cerlls. Start with spherical cells.

[bjpalmer]

The mechanical interactions between particles appear to be implemented in the BMXParticleContainer::EvolveParticles method located in the src/des/bmx_pc.cpp file. Most of the existing interactions appear to be commented out, but this has code for evaluating pairwise interactions.

[bjpalmer]

@asalmgren added a EvolveParticles routine for evolving particles in des/bmx_pc_interaction.cpp that acts as a scaffolding for creating a particle interaction functionality. This routine seems to be similar to the EvolveParticles routine in the bmx_pc.cpp file so I commented out the version in bmx_pc.cpp. I also created a BMXCellInteraction class that is located in chemistry/bmx_cell_interaction.H and chemistry/bmx_cell_interaction.cpp that is called by the EvolveParticles routine. This class is designed to encapsulate the particle interactions so that they can be more easily modified in the future.

[bjpalmer]

Implemented a simple cubic interaction force between particles. This is repulsive at short distance and slightly attractive at longer distances up to a cutoff value. It vanishes at the cutoff. Test cases indicate that the particles start moving when a particle splitting event occurs.