pilabels

Automatically replaces xticklabels with rational multiples of pi.

- picks reasonable limits and ticks
- uses latex for pretty fractions if necessary
- replaces old labels produced by previous calls
- does NOT autocorrect on zoom/pan/resize

algorithm:
- choose limits as smallest n such that pi/n < x_data_range
- optional: increase n to next smallest "round" number
- "round" means "power of 2" right now; could be a*b^n with a<b, 2^a*3^b, etc
- choose step size as smallest d that ensures at least L tick marks

I'm sure there are plenty of bugs