Provide KDocs for public API: Start

[Jolanrensen]

Before March 30th, we would like to have a good base of KDocs in the public API so we can continue documenting the other functions.

A good exploration start was made in this PR, but several things need to be done in order to get the KDocs in the master branch.

• Clean up KDoc Preprocessor Gradle Plugin to be reviewed.
• Have a review of KDoc Preprocessor Gradle Plugin by someone else so it can more safely be included in the master branch of DataFrame.
• Merge Enable Doc processor plugin #308 into master

Until this is finished, all KDoc contributions should be done to the jpc-kdoc-reuse branch.

In terms of documentation I'll start with a list of some of the most common API functions and who will take care of them (Feel free to propose changes/removals/additions).

Jolan @Jolanrensen :

• documentation
  • DocumentationUrls.kt (will be expanded when needed)
  • AccessApi.kt
  • ExpressionsGivenColumns.kt WIP
  • ExpressionsGivenDataFrames.kt
  • ExpressionsGivenRow.kt
  • SelectingColumns.kt
  • SelectingRows.kt
  • Other common concepts if needed
• update.kt
• Nulls.kt (fillNulls, fillNaNs etc.) WIP
• ColumnsSelectionDsl.kt
• select.kt
• add.kt
• remove.kt
• group by?
• TBD

Aleksei @zaleslaw:

• filter.kt
• Operations that use SelectingRows.kt (@zaleslaw might be easier to work per file in most cases actually, except for GroupBy.x() of course, wdyt?)