Add support for probe files

[shashwatsridhar]

SWAN currently assumes the geometry of the probe to be that of a Utah array (96x96 grid of electrodes) and this is the geometry that the channel selector in the GUI displays. Since the channel selector is the only way to choose multiple channels, it becomes imperative that the user gets a chance to specify the geometry of the probe. This is best done through probe files (.prb) since these have already been implemented in phy and its derivative packages.

The idea would be to allow the user to choose a probe file (and hence a custom geometry) that is then reflected in the channel selector as well.

[mdenker]

See also here for a definition of prb files.