Add imputation operation for neighbours as in MissingDataGUI

[njtierney]

Add imputation operation for Neighbours, as in MissingDataGUI - Line154

[njtierney]

There are actually two methods applied - these are:

• impute_knn_avg, and
• impute_knn_rand

These should take arguments, .tbl and k.

[njtierney]

These still need some work, I need to add:

• [ ] proper handling for character/factor variables
• [ ] split impute_knn into _avg and _rand functions
• [ ] Look at adding _at and _if scoped variants

[njtierney]

Check out some similar imputation strategies in the same family here, as well as an interesting blog post here

[njtierney]

This should be covered by #173