Save data

[dpizetta]

We need to implement a save function that save the data recorded in the RecordingPlotter (the sub set of real time plotter). May we can use the numpy function savetxt and loadtxt (like .dat format, in columns). I think this is the easiest solution.

Also, we need to check the recorded file before begins a new one, or when the program closes, etc.

[dpizetta]

Done.
Needs more testing.