Use package lintr in unit tests

[Guillawme]

See https://github.com/jimhester/lintr Useful to enforce a consistent coding style automatically.

[Guillawme]

I tried to set up automatic lint checking as part of testthat tests, following the instructions from lintr README.md.

Several problems here:

• Current CRAN version 1.0.2 doesn't ignore excluded files, so I tried the latest GitHub version (jimhester/lintr@38b044e) which is reported to have fixed this issue (jimhester/lintr#304).
• Current master (jimhester/lintr@38b044e) doesn't work when a .lintr file is present (see jimhester/lintr#268).

As a result, I must pass options directly in the unit test function instead of via the .lintr file, which is a bit of a mess...