Clean up package related code

[thomas-muench]

For piccr to be a proper package we should update the following things:

• [x] update/add function documentation.
• [x] remove all library(<pkg>) statements at the top of the source code files; instead, put all necessary external packages in the Imports section of the DESCRIPTION file and either use the :: syntax when calling external functions in the code or import them into the package namespace (e.g. suitable for the magrittr pipe operator); see Hadley's advice on these issues here and here.
• [x] convert the dataProcessingGuide.Rmd to a vignette for the package and update it.

[thomas-muench]

Note for this that the used functions lag() and filter() are from package dplyr, not from base package stats.

[thomas-muench]

First two points handled by #50, third point handled by 6ff6108.