add single wf visualiser

[MattZur]

Adds to packs the choice of vis. This allows the mule hdf5's to be visualised one waveform at the time.

Currently, the only option is waveform as in single waveform visualiser. Other options such as average, smoothened single waveform, and FFT will be added later on.

This initiates a GUI with a scrollable element that allows for the waveforms in the mule hdf5 to be visualised one by one.

[jwaiton]

First comments from a user perspective before I dive into the code a bit more closely:

• The example config (packs/config/vis_wd1_1channel.conf) should point by default at one of the data files in packs/tests/data/.
• Due your sampling rate, using plt.plot(..., marker = 'o') makes waveforms with larger sampling size look like so:

So changing it to marker = 'o-' would resolve this

• The visualiser works on 'waveform number', but for files with multiple channels (WD2), how does this correlate? The visualiser should relate channels and events to the relevant waveform for this case. For one-channel cases, it can just say 'channel 0' :)