Rename voeparse->voeventparse

Although slightly more tedious to type, it's clearer to match the import
to the package name (modulo hyphenation).
If starting afresh I would probably drop the hyphen in the package name too,
but I don't want to break all the external links this point,
and anyway it reads better.
Rename 'set_where_when' -> 'add_where_when'.

Since a packet may have more than one entry under the 'WhereWhen' section.

Similarly, add an 'index' parameter to the pull_<blah> functions acting
on WhereWhen.

Finally, add some more details to the lxml gotchas section.