Bug in handling nudge that is a "long" vector

[aphalo]

The most logical approach with the nudge argument would be to accept numeric vectors of any length and recycle them. One case works after today's edits correctly with three position functions: position_nudge_keep(), position_nudge_to() and position_nudge_center(). This case is when x or y are longer than 1, but shorter than the number of rows in data. In this case not only recycling needs to be done but also reordering so as to support the frequent use case of nudging labels alternatively in opposite directions or by different distances. This is an effective approach to reduce overcrowding.

Another case that needs to be handled with a warning is when the x or y vectors are longer that rows are present in data. This needs to be implemented in all the nudging position functions.

This needs to be also implemented in the remaining position functions that support nudging.

[aphalo]

This is now implemented and unit tests added. However, when a factor is mapped to one of x or y the ordering does not work as expected. I put this on hold for now.