Add "priority date" legend option

[amabdallah]

Use case: in a river basin of interest, select a single-priority date, then color code all the PODs junior to it in red and senior to it in blue. I'll add more details later

https://fwp.mt.gov/binaries/content/assets/fwp/conservation/fisheries-management/water-management/fwp-water-right-call-protocol-final-7-22-22.pdf