Method to get currently active columns.

[inferrna]

Currently, we have methods to manipulate columns (add, insert, remove), but there is no method to retrieve a list of current columns.
For example: if I want to remove column Count from this table , or swap Count with Rate, at this time I don't see any way to do it, except keeping table structure in a separate vector.
In case of above example, it would be nice to have a method .columns() returning Vec<BasicColumn>.