Update GUI for custom elliptical pipes

[cbuahin]

GUI should only either accept rise or span. The other can be calculated use S/R factors.

This issue is related to [this one] (USEPA/Stormwater-Management-Model#144) raised by @LRossman

[cbuahin]

Leave it up to users to specify rise and span correctly.

[cbuahin]

I set up a quick experiment using a 100ft pipe with 10ft sub-sections and slopes of 1% and 5% to see the impact of using the constant S/R ratio of 1.56 versus the correct ratio on depth and velocities given the same flows. The results showed some differences. There were larger deviations for the smaller pipes than for the larger pipes as one would expect.

Also, the deviations tended to be larger when depths approached the crowns of the pipes, which is also to be expected. Finally, the vertical ellipses showed larger deviations in depth than the horizontal ellipses as changes in flow lead to relatively larger changes in depth.

In light of these differences, it makes sense to me to keep the status-quo. Users have the option of selecting one of the standard sizes or they could prescribe their own rise and span values which will lead to more accurate and precise calculations.