Draft: AST-1603 Remove start channel from DPInfo

MSReader would always set the start channel to 0, since itsStartChan was still 0 when creating DPInfo. Since MSReader can internally handle the logic around the start channel, it suffices if DPInfo has the channel frequencies and widths but not the start channel index in the original input MS.

Also increase the minor version to 6.2.0, since the Python API changed.