Added more functional programming like versions of ssu and psu
Added TODO to agg function
Modified psu to correctly calculate np_freq and fixed up comments
Fixed up comments in ssu
Modified psu to include protected status
Added stratData function to calculate weighting per stratum
Modified strat to calculate stratum level density and/or occurrence
Fixed merge conflicts

Merge branch 's3classes' of github.com:harryganz/rvcstats into s3classes

Conflicts:
	rvcstats/R/psu.R
	rvcstats/R/ssu.R