calc_rad appears to return an offset list of radii

[EzryStIago]

The "rad" variable returned by calc_rad is off by one radial bin width.
E.g. if we have bins 0 to 20 with dr=5, calc_rad SHOULD return [2.5, 7.5, ... 17.5] but actually returns [7.5, 10,... 22.5]
Temporary fix: subtract one bin width before plotting