Merge branch 'develop'

bfps/NavierStokes.py

bfps/cpp/interpolator.hpp

bfps/cpp/interpolator_base.hpp

bfps/cpp/particles.hpp

bfps/cpp/particles_base.hpp

bfps/cpp/rFFTW_interpolator.cpp

bfps/cpp/rFFTW_interpolator.hpp

bfps/cpp/rFFTW_particles.hpp

bfps/fluid_base.py

done.txt

setup.py

tests/base.py

tests/test_convergence.py

tests/test_frozen_field.py

tests/test_interpolation.py

tests/test_io.py

tests/test_particles.py

tests/test_plain.py

tests/test_scaling.py

tests/test_time_step.py