add dplyr operations

[ocramz]

• [ ] mutate() adds new variables that are functions of existing variables
• [x] select() picks variables based on their names.
  • see the text, int etc. lenses
• [x] filter() picks cases based on their values.
• [x] summarise() reduces multiple values down to a single summary. ~~- [ ] arrange() changes the ordering of the rows.~~ not needed