Add QCodes data file support

[herrkeuner]

This adds support for QCodes-generated data files. Main difference to QTlab files:
all meta-info of the measurement is stored in a json-file, including most of the info that is directly written into the .dat file header for QTlab files.

I tried to keep the impact on the existing code minimal, and ended up doing it by
expanding the "filetype" dictionary to a class, which allows for runtime determination of the .dat file-type (QTlab or QCodes). I also wrapped the gzip-class into a factory function, so it gets the correct parent class at runtime.

for handling the qtlab or qcodes .dat file parsing there are now two child classes of dat_parser that mainly differ in handling the retrival of the info for the "header" variable handed over to the Data class.

Additional minor changes:

• changed the default "legend" boolean in the plot2d function in the plot module from True to False. It just produced warnings all the time that it didn't actually have a legend handle.
• in the view module, the file path now only gets shortened if the code is actually executed under windows
• Moved the getsetfile function from the tessierview class in the view module to the new filetype class in the data module to avoid circular dependency between the two modules (which for some reason is only a problem if you run the code under python 2.7)