Removing constraint on additional properties on generic track data
as this prevents the domain specific metadata to add their fields.
Adding species and assembly as required properties.
bigDataUrl is not required since a composite track can be defined
in terms of the more basic ones, and it can actually miss the URL.
Preventing additional properties on the hierarchical tracks so that
the validator, once it has chosen to follow a certain path (i.e. the
specification of a given hierarchical track), does not try to follow
other specifications.