modeling type: vector - add N items

[hogi]

☑ cool new feature
☐ could help many users!

☐ removes a bug

☐ nice to have

☐ nobody needs it

What inspired this wish list request? 

Thanks for implementing the modeling type Vector for Columns with data type expression.

Thanks for the aggregation options which are provided in the new formula column menu:

What is the improvement you would like to see? 

Please add more options:

- N (how many entries are in the vector)

- N non missing 

- N unique

- @ community: do you have further suggestions?

Why is this idea important? 

the new options will make the modelling type Vector more useful.

other wishes from 

[hogi]

another option:
- robust version of to columns:
  -- doesn't complain if a cell is still empty

  -- doesn't complain if a vector in a cell has less entries i than the vector in row #1

Implemented via AddIn -> no need anymore