Stack single-offset sections

[EvanBianco]

Currently, we are showing single-offset stacks of of the spatial cross-section. I anticipate that users will want to create "range-limited stacks", and "full-offset stacks". For instance, a near-offset stack and a far-offset stack are useful for Shuey type intercept and gradient computations.

I can think of two potential design solutions for the interface:

1. replace the offset angle slider with two sliders: first offset angle, and last offset angle. Make them so that first can't be bigger than last. First equal to last is the situation we currently have.
2. place two blue circles on the sliders, if that is at all possible. Prevent them from crossing (if needed).