add nd_geolocation_dbxref table

[nmenda]

create table nd_geolocation_dbxref (
nd_geolocation_dbxref_id bigserial not null,
nd_geolocation_id bigint not null,
dbxref_id bigint not null,
is_current boolean not null default 'true',
primary key (nd_geolocation_dbxref_id),
foreign key (dbxref_id) references dbxref (dbxref_id) on delete cascade INITIALLY DEFERRED,
foreign key (nd_geolocation_id) references nd_geolocation (nd_geolocation_id) on delete cascade INITIALLY DEFERRED,
constraint nd_geolocation_dbxref_c1 unique (nd_geolocation_id,dbxref_id)
);
create index nd_geolocation_dbxref_idx1 on nd_geolocation_dbxref (nd_geolocation_id);
create index nd_geolocation_dbxref_idx2 on nd_geolocation_dbxref (dbxref_id);