Make private class in DV reader public (see #12358)

In order to be able to handle the DVExtHdrFields as anything
other than a string, the object will either need to be made
public or be made to implement some public API (e.g. Map).
Is this the cause of http://ci.openmicroscopy.org/job/BIOFORMATS-5.0-merge-full-repository/776/console failure?