Consider making LaserFrame subscriptable to be data driven

[clorton]

It could be useful to access a "column" in a LaserFrame from data, e.g.

frame.agents[propertyname] # Where propertyname is a string, e.g. "age"

A more verbose alternative (admittedly Pythonic, but clunky) is the following:

getattr(frame.agents, propertyname)

Basically, LaserFrame.__getitem__(propertyname) would call getattr() so that laserframe[propertyname] would work. We would not implement LaserFrame.__setitem__() as this should be done through add_scalar_property() etc.