AST-1603 remove MSReader::itsStartChan
Use a local variable instead of itsStartChan.
Throw an error when using startchan/nchan with multiple MS's:
- Otherwise those settings would apply to each input MS. A setting per MS would be more logical, since input MS's may have varying numbers of channels.
- When concatenating channels of different MS's, you typically want to select all channels in each input MS.