implement lead and lag column functions

[amorphous1]

I had a go at implementing lead and lag as suggested in PR #85. I did not include parameter "default" from the dplyr reference, because I was not sure how to do that sensibly within the static type system. (The various "is StringCol" / "is DoubleCol" etc. checks feel somewhat duplicated, I wonder if the base class DataCol could somehow have a reference to the values array?)

[holgerbrandl]

Thanks, I added a default argument.

I've also reimplemented pctChange to use lag (which looks really pretty now). Though, pctChange still needs a fill_method parameter to enable NA handling. I've slighted adjusted the test definition to be more precise here.

Indeed, the column type dependent handling in many krangl methods is tedious, but I think there is no real way around it. Feel felcome if you have an idea how to do.