Built-in shapefiles

[slucey]

Right now users need to read in a shapefile then provide it to the function calls. Most of the time we are using either the NEFSC survey strata or the EDAB EPUs which are included in the package. I suggest changing the code to use a keyword (i.e. 'strata' or 'epu') to load in the shapefile as part of the function call rather than separately.

[andybeet]

Something like...

Let the areaPolygon = strata or epu or an sf object. Then in swept_Area_biomass function check for sf class, something like: any(class(area)=="sf")

if FALSE then, check for string = strat or epu, read in built in shapefiles and set appropriate areaDescription if TRUE, then use shapefile passed and areaDescription

we may need to add a bunch of error checking into this function, and provide useful error messages

we should probably also set areaDesription =NULL as default too.