Fix RFieldBase::GetNElements() for record/class fields

[jblomer]

Check duplicate issues.

• [x] Checked for duplicates

Description

For all but record and class fields, the number of elements of that field is given by the number of elements of the principle columns. Records/classes don't have columns of their own. The information must be retrieved from one of the subfields.

Reproducer

The problem arises, e.g., when trying to iterate over the field range of a view of a record item of a collection.

ROOT version

master

Installation method

n/a

Operating system

n/a

Additional context

No response

[jblomer]

In addition, enum fields and atomic files have no attched columns.