PipeOpRowApply

[advieser]

Closes #318

New PipeOp inheriting from PipeOpTaskPreprocSimple.
This works by calling apply on the task data with FUN = applicator (param of the PO) and simplify = TRUE. This should return a matrix or a vector. If a list is returned, an error is thrown.
The rest of the code is there

• to handle the differences between vectors and matrices returned by apply.
• to create column names automatically, that might have been dropped by apply (depends on the applicator).
• to handle tasks with zero rows.

Tests might be a bit too excessive (?), I tried to cover all the points I talked about with @mb706.