Add linting to continuous integration

[matt-dray]

Close #8. Note that:

• this is vanilla linting with {lintr}, but I've added a .lintr file where we can set or override defaults as needed
• I've deactivated the object-usage linter in the .lintr file because it gave a false positive for a function name that's actually part of the package
• I've fixed all the lint failures as far as I can (and kept these in line with the tidyverse style guide if needed), though some are still outstanding:
  • we have an argument called F (fails the object-name and T-and-F linters)
  • many test files have commented-out code awaiting some input error handling (see Add function input checks #28)

[ThomUK]

All looks good to me. Sorry for the delayed review. Feel free to merge and if there are any revisions we can mop them up separately...