when starting experiment, we should be able to provide an arbitrary field for description

[qgeissmann]

This should be only a string 'a-z0-9-'.

This will be:

• stored in metadata for tracking
• inserted in the filename for video

Devices needs to check this string only contains allowed chars (raises an exception if not)

This should be v. useful for people who run several experiments on the same day.

[qgeissmann]

@nazabic is this fixed then?

[qgeissmann]

...