Add octree and new models

[limefax]

• Generalised the code to N dimensions
• Can be compiled with e.g. make gcc D=3 to get ./nbody_d3_gcc
• Removed old solar model
• Added a uniform model (that works for any dimension, not particularly meaningful)
• Added 3d galaxy animation
• Updated the Plummer model (which was meant to be 3d in the first place)

Some very inaccurate benchmarks run on my laptop:

$ ./nbody_d2_gcc -s 5 -n 1000000 && ./nbody_d3_gcc -s 5 -n 1000000 && ./nbody_d4_gcc -s 5 -n 1000000
Starting simulation
Done simulation
Total time: 4987.50 ms
Starting simulation
Done simulation
Total time: 20318.35 ms
Starting simulation
Done simulation
Total time: 70339.24 ms

$ ./nbody_d2_gpu -s 5 -n 1000000 && ./nbody_d3_gpu -s 5 -n 1000000 && ./nbody_d4_gpu -s 5 -n 1000000
Starting simulation
Done simulation
Total time: 2679.34 ms
Starting simulation
Done simulation
Total time: 10816.77 ms
Starting simulation
Done simulation
Total time: 63003.19 ms

For both 2D cpu and gpu I get similar performance as before.

[gonzalobg]

This looks great!

[gonzalobg]

Thanks, lets merge this!