made vi module compatible with TCPIP interfacing & current pyvisa/NI-VISA

- PNA class now requires the complete VISA address (including interface
identifier) as an argument
- New version of pyvisa and/or NI-VISA shows changed termination
character behavior that causes problems when retrieving data. Set the
termination character manually to \n for now. Should work with both
GPIB/TCP on Win and Mac. To be tested more thoroughly!
added a few PNA functions (gating, normalisation etc.)