Allow references containing HETATM in `caprieval`

## **Desired feature/enhancement**

It would be nice to be able to use reference containing HETATM in `caprieval`

## **Motivation**

For now, `caprieval` module does not accept HETATM residues during the comparison

## **Description**

Can be:
- always accepted
- in the form of a new parameter: `keep_hetatm = true`
