Add the ability to combine several consecutive segments in one record

## Background
It is quite a common pattern in mainframe files that large records are split between several segments.
Cobrix can combine hierarchical records where one parent record can have several child segments. This creates an child array of struct types for each record.

But Cobrix does not support the case where the child segments can be encountered only once, thus eliminating the need of the child array.

## Feature
Add the ability to specify which segments should be combined into one record.