Make physics classes separate libraries

[neiser]

In order to reduce compile time of the main program, provide each physics class as a shared library, possible only a factory method to create it. Then Ant loads that library, gets the factory maker and creates the physics classes as usual. This replaces the PhysicsRegistry.

The cmake build system could prove helpful here, and it's strongly correlated to #33.

Apply this scheme also to plotters and Ant-plot.

Useful resource for dlopen: http://www.linuxjournal.com/article/3687 It mentions that __init mechanism is broken. Check if that's still true.

[osteffen]

Wrote this. C++ library loader
(Of course this exists already elsewhere... was just interested in learning)

Could be used to get a factory method for physics classes in the libs.