fix R CMD check problems

[achubaty]

currently, travis builds fail due to an error* related to missing arguments in extract ([) methods.

* checking PDF version of manual ... ERROR
Rd conversion errors:
Error : /home/travis/build/PredictiveEcology/NetLogoR/NetLogoR.Rcheck/00_pkg_src/NetLogoR/man/extract-methods.Rd:51: no comma in argument list following \S4method

Currently, extract-methods.Rd:51 reads:

\S4method{[}{NLworldMatrix,missing,missing,ANY}(x) <- value

the problem seems to be the missing arguments . see https://stackoverflow.com/questions/31271406/s4-documentation-of-with-missing-arguments

---

* (this is a warning on my system, but I've set up travis to treat warnings as errors)

[achubaty]

code coverage commits were accidentally tagged as belonging to this issue but they belong in #19