Rename *TDigest pointer receiver from 'cs' to 'd'
Rename *TDigest pointer receiver from 'cs' to 'd'

As a follow-up to 0cfad43ad14873b8b3bd0d629713323b7ce1e296, this
commit renames all the receiver variables in TDigest's methods to
something that makes more sense.

spenczar/tdigest