create rwd_agg class

[rabutler]

for use by the new tbl versions of the get/process data functions

• [x] also create validate_rwd_agg() that will check and see if the user specified functions exist and that sort of thing
• [ ] need to be able to specify rwd_agg by reading in a csv file. Right now, it's two steps, but we could make it part of the constructor? (now #70)
• [x] tests
  • [x] see if rbind works for the rwd_agg; it should work, but we want to throw an error if the variable names get reused