viewport-source and iteration-source element names

I think it would be nice to rename 'viewport-source' and 'iteration-source'
to
just a plain 'source' element.  The 'source' element specifies an input
where the context (or port) is assumed (e.g. the target of the viewporting
or iteration).

This means we have:

<p:viewport name="foo" match="/bar">
<p:source>
   <p:pipe .../>
</p:source>
...
</p:viewport>

and

<p:for-each name="foo">
<p:source>
   <p:pipe .../>
</p:source>
...
</p:for-each>

-- 
--Alex Milowski
"The excellence of grammar as a guide is proportional to the paucity of the
inflexions, i.e. to the degree of analysis effected by the language
considered."

Bertrand Russell in a footnote of Principles of Mathematics