grid api

Why are we worried about this at all:

`grid['electric monkey count']`
`grid[10]`

is there a danger in being loose like this? i could see why we'd like to avoid '.' notation as we don't want users to overwrite column names... but otherwise i think it's okay, they this is consistent:

for name ['time', 'x', 'y']:
    print grid[name]

for index in range(10):
    print grid[index]

We just have to restrict names to being *strings* and then there's no ambiguity.