get_trace suffers from timeout error

[mgiustina]

Function get_trace in agilentN5242A.py tends to fail with VI_ERROR_TMO: timeout expired before operation completed.

I haven't identified all of the configurations in which this error does/doesn't appear.
The error appears when user requests a trace that wasn't already on the screen. For instance, if the screen shows S21 on trace2, then
cxn.pna_x.get_trace()
will succeed, but
cxn.pna_x.get_trace('S11')
throws the error.

Each time the error appears, all subsequent calls to get_trace fail. It seems to be necessary to restart the pna_x server in order to clear the error.