Merge pull request #8 from Galvant/dev

Pull from upstream
Added new file communicator class.
Fixed *OPC? query in generic_scpi.
Merge pull request #9 from Galvant/dev

Started adding SCPI function generator
Refactored amplitude setting to handle generic units and mnemonics.
Merge branch 'dev' of https://github.com/cgranade/gpibusb-comm_code into dev
Fixed bugs in previous refactoring.
Merge pull request #10 from cgranade/dev

Bug fixes and file communicator class