Regrider

Downsample 3D cartesian grids using FFTW.

Natively handles gbpTrees and VELOCIraptor files.

TODO

• Malloc orig using FFTW (with inplace padding)
• Do an inplace FFT
• Convolution
• Inverse FFTW
• Reshuffle data inplace
• Write the data back out
• Get rid of choice of grid and convert all grids
• Documentation
• Do an implementation for VELOCIraptor
• Proper tests for known solutions