#172 volume element for publications

[Ivan Tubert-Brohman]

For publications such as journals and multi-volume
books it is often necessary to include the volume
number. Something like this should work:
<volume>123</volume>

It would be an optional child of <pub>. There are
various styles for formatting journal references, but a
common one is like this:

Doe, J.; Smith, J. On the addition of a volume element
to XMLResume. Journal of Resumenomics *2005*, /123/, 57-69.

Which comes from

<pub>
<author>Doe, J.</author>
<author>Smith, J.</author>
<artTitle>On the addition of a volume element to
XMLResume</artTitle>
<bookTitle>Journal of Resumenomics</bookTitle>
<date><year>2005</year></date>
<volume>123</volume>
<pageNums>57-69</pageNums>
</pub>