Add support for phyloreferences with multiple external specifiers

[gaurav]

At the moment, we support phyloreferences with a single external specifier, since we can easily create a definition for a node that:

1. has descendants for each internal specifier, and
2. excludes lineage to one external specifier

I'm not sure if we can extend this to multiple external specifiers, since I think each "excludes lineage to" will identify a different pinning node. We should try it and, if unsuccessful, should develop a method for resolving such phyloreferences. Examples of such phyloreferences are included in Carvalho-Sobrinhoa et al, 2016, which can be used as a test case.

[hlapp]

I'm not sure if we can extend this to multiple external specifiers, since I think each "excludes lineage to" will identify a different pinning node.

Perhaps if taken on their own. But surely the semantics here are conjunction, no? So an additional external specifier can make the identified clade smaller but never larger.