Write basic interface for dark matter particles only

[felicityguest]

Using Integrate a C++ code as a guide:

• Break down Arepo main.c into various functions (which initialise, run main loop and finalise)
• Write basic Python interface for AMUSE
• Amend machine-generated C interface for AMUSE

[stephenpcook]

Progress made in interface getter functions in d9f233d and 817000a.