BUG: use size_t for conversion to numpy array

std::accumulate was implicitly using unsigned int as a template
parameter. The integer is not explicitly a size_t to ensure the type
is large enough.

Change-Id: I2b078f39cea0882126b7739c18557e85fde74307
STYLE: fix indentation style

Change-Id: Id079e342331fa5e1ed4333041707e9bbe0f080df
DOC: correct label statistics bounding box definition

Change-Id: I58a709c0f68be897dd594012f3c5f3a4a287628a
DOC: update description of label statistics image filter

Change-Id: I806e0796d820fcb01c56896ead8429559fcddb13
Merge branch 'SIMPLEITK-421_FixStatisticsDoc' into release

Change-Id: I1639be79a93aeb0438ba1a9190841f6d5cb34eea