better handling of raw 4 year and 5 year grad rate

[almartin82]

ex1 <- get_grad_rate(2013, '5 year')

 school_name grad_rate
1               ATLANTIC CITY HIGH SCHOOL    0.6716
2 ATLANTIC COUNTY INSTITUTE OF TECHNOLOGY    0.9781
3              BUENA REGIONAL HIGH SCHOOL    0.8480
4         EGG HARBOR TOWNSHIP HIGH SCHOOL    0.9300
5                    ABSEGAMI HIGH SCHOOL    0.9121
6                    OAKCREST HIGH SCHOOL    0.8407
  five_yr_grad_rate end_year methodology
1             73.40     2013      5 year
2             97.06     2013      5 year
3             87.15     2013      5 year
4             94.18     2013      5 year
5             91.45     2013      5 year
6             86.01     2013      5 year