Basic FCS3.1 support.

Hi guys,
we are starting to get some FCS3.1 files, which we need to analyse. I used to just manually change the header to FCS3.0, but it seems cleaner to add support for FCS3.1 to flowCore. It is very straightforward as FCS3.0 and FCS3.1 are basically the same as far as flowCore is concerned. There are some additional optional keywords, such as $PnD or $SPILLOVER defined by FCS3.1, but these don't seem to be used much yet, and it shouldn't really affect how the FCS file is read by flowCore anyway.