Add Upstream distance input parameter for the NLDI tool

[amabdallah]

1. Let's always set the downstream distance to 999 (which means go to the bottom of the river no matter how far). Downstream data are very small, so that's okay.

2. Upstream distance is tricky, so let's let the user choose how far they want to go upstream through an input box. By default, the parameter value should be 500. Users can edit it. Next to the box, we need to add a question mark with a hover-over text display: your computer may be slow to handle data for more than 500 KM upstream distance.