MSO: Store TTAGs

Add option to store event TTAGs.

All TTAGs can be retrieved in one go with :WAV:XLISt? query. Format is either binary (float64, 8 bytes) or plain text (%.14e), and gives the time of screen start (not trigger time, but the trigger - start shift is the same of all events) in seconds. Warning: the waveform is transferred in WORD format which is incompatible with TTAG transfers (so transfer method must be set before each WF or TTAG readout).

In case of plain text, each time stamp is of the form 4.03865939007923E-02 = 20 characters (+ 1 for EOL/separator). Given the plain text format can be quickly human-read in a file and that its size is only 2.5 times as large as the binary (21 instead of 8 bytes), I suggest going for plain text.

Note: the size is negligible compared to the actual data points anyway (thousands of int16 = few kB per waveform).