Removed psu and rvc summary functions and docs
Added 3 functions rvcData, ssuDensity, psuDensity, and some documentation
docs added for rvcData and ssuDensity
Added documentation for psuDensity
Also, slightly modified documentation for other functions
Modified doc for ssuDensity to link to psuDensity
Fixed aliases in documentation so links work properly
Added function stratDensity
Minor change to ssuDensity documentation
removed orphan {}
Made ssuDensity return data.frame instead of RVC
Removed reclass of ssu to list in psuDensity
ssuDensity now returns data.frame so reclass not neccessary