documentation and checking

[richardjtelford]

I recommend writing the documentation as you go rather than planning to write it all at the end. It doesn't need to be wonderful documentation immediately, just the parameters and an example.

Also, I recommend regular checking of the package with devtools::check() or the button in the build tab. There will always be lots of issues, and it it better to deal with them one at a time than trying to fix them all at the end.