Parse 'ActiGraph' 'GT3X'/'GT3X+' 'Accelerometer' Data

Implements a high performance C++ parser for 'ActiGraph' 'GT3X'/'GT3X+' data format (with extension '.gt3x') for 'accelerometer' samples. Activity samples can be easily read into a matrix or data.frame. This allows for storing the raw 'accelerometer' samples in the original binary format to reserve space.