Expose hydropower calculation variables to the user

[gassc]

• A: calculated from watershed delineation GP service, but should be tweak-able by the user
• h: calculated from the elev profile GP service but this could be messy/inaccurate if the topographic data is coarse
• x: environmental flow requirement (default is 0.3, range options from 0.1-0.5)
• e: efficiency. default is 0.7, but they can put in whatever they want.

[gassc]

For x, e, consider using a slider for controlling value.
A will just need to be a text box. Consider making units selectable.
h will also need to be a text box, initially, but could be derived from adjustment of the Elevation Profile visual later on if needed.

[gassc]

These are now exposed through the API, via 85b9524