update American river rearing habitat to the Water Forum flow to WUA relationship (currently used in VAs)

[x] Use American Rearing WUA from VAs in place of IFIMWUA for rearing habitat in DSMhabitat in data-raw/watersheds/american/american_river.Rmd - Refer to VA processing to see how they got data into flow to acre format.
[x] Update both inchannel rearing and floodplain rearing
[x] existing data on American is in WUA sqft/1000 ft. New data is in WUA acres, use deer as an example for what WUA acres should look like. https://github.com/CVPIA-OSC/DSMhabitat/blob/main/data-raw/watershed/deer/deer_creek.Rmd
[x] Saved data object should be in sqm (use deer as example) for inchannel rearing, I think acres okay for floodplain, confirm on deer creek
[x] Duplicate juv data to work for both Juv and Fry...erin will check for what we have done in this case in the past
[x] Make plots that are x - flow to y - area (acres)
[x] Update documentation to describe change in data, list new data sources.
[x] Rerun markdown to save updated data in american_river.rmd
[x] update DSMhabitat::watershed_methods to change from wua to sqm (check current nomenclature used in the file)
[x] Rerun data-raw/cache_habitat.R file to resave new American river data.

CVPIA-OSC / DSMhabitat