entryGroup and additionalEntry

[seanredmond]

Adds to mechanism for dealing with repetitions:<entryGroup> and<additionalEntry>.

<entryGroup> is for cases where a single author has multiple <copyrightEntries> and simply acts as a container for the entries:

<entryGroup>
    <author><authorName>ADLER, MORTIMER J.</authorName></author>
    <copyrightEntry>
    <copyrightEntry>
</entryGroup>

<additionalEntry> is for cases where a single title as continued by parts that have their own registrations (most likely issues of a periodical)

<copyrightEntry id="GUID6" regnum="BB21264">
    <title>THE ADVENTURES OF HAP HAZARD</title> <note>(in The
    Co-operator)</note> <author><role>Appl. author</role>:
    <authorName>Jack Hamilton</authorName></author>. ©
    <publisher><pubName claimant="yes">Employers Casualty
    Co.</pubName> <pubDate date="1962-07">Jul62></pubDate></publisher>
    © <regDate date="1962-07-12">12Jul</regDate>;
    <registrationNumber>BB21264</registrationNumber>.
    <additionalEntry regnum="BB21524">
        <pubDate date="1962-08">Aug62</pubDate>
        © <regDate date="1962-08-15">15Aug</regDate>;
        <registrationNumber>BB21524</registrationNumber>
    </additionalEntry>.
    <!-- more additionalEntry elements -->
</copyrightEntry>

Note, the containing <copyrightEntry> has the the first registration number. Whatever needs to be repeated in each <additionalEntry> can be repeated.

Fixes #16

[hadro]

Yep, that all makes sense!