Rewrote @species

The first version generally did what it was supposed to, but was overly
complicated and brittle, because it basically did its own parsing.
This version simply wraps the user code in a function and uses @locals
to access the ready-parsed variables. This is not only much cleaner,
but also gives the user more of what he expects (e.g. the ability to
reference previously defined variables).

I had actually tried this approach before, but only got it to work
once I understood how @locals works under the hood, plus a lot of other
Julia internals. So I now not only have more elegant code, I also
learnt quite a bit - a good win :-)