Add placeholder for missing roxygen documentation

* R/diff.r: Add placeholder :TODO:DOCUMENTATION: for missing roxygen
  slot documentation of S4 classes

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Rename Makefile doc target to roxygen

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>
Initial work with reset functionality

* Add and update files for adding reset functionality

Signed-off-by: Stefan Widgren <stefan.widgren@gmail.com>