Add method to update data from a JGI domain

[hbsmith]

for example...

Jgi.update_domain(self, path, domain, database='all', 
                                 assembly_types = ['assembled','unassembled','both'])

...would cause an existing directory (as specified by path) to be updated with all domain organisms which are part of the JGI domain, but not currently stored in the directory. The method should check to make sure the directory of the path matches the directory of the domain as a weak way to verify the user isn't trying to update a directory from a different domain. Of course, the user could still update the directory with organisms from a different database (eg jgi instead of all) than what it was originally, or with different assembly types.