DasAxis should allow TickLabeller to be set

[jbfaden]

I ran across the tickLabeller object which the tick curve renderer uses to label each tick. This is trivial to use with the normal DasAxis object too, but this was never coded.

I've added a tickLabeller property to the DasAxis which should have a trivial effect on performance.

[jbfaden]

You can now set a TickLabeller, but the bounds drawn are not used when calculating the bounds of the axis. (This is also true for the TickCurveRenderer, and will be done there too.)

[jbfaden]

Each plot now has its own graphics context, so that it's easy to rotate ticks. I need to check to see if there's a non-trivial effect on performance and editability of plots.