Add LN2_PYRAMID_RESAMPLER #116
Description
I often need a quick and simple pyramid resampler (2x 3x 4x downsampling). I should implement a mean-pool resampler. This will be useful to investigate coarse patterns in very large fMRI data (>10 GB), and also helps with SNR with averaging. I want to keep it very simple averaging and not Gaussian smooth + downsample on purpose.