[DAS2] modification to /type request & response

[Ed Erwin]

Adding the "namespaces" query seems reasonable to me.

But additionally, how about /das/genome/type returning only those types 
for which the server actually has some existing annotations, not all the 
annotations it "knows" about.

Allen Day wrote:
>
> Second, making a /type request is, according to the spec, supposed to
> return all types known to the server.  If we allow non-sofa-inheriting
> types into the datasource, the result can be a *very* large document.
 > ..........
> 
> I'd like to be able to restrict a /type response to a subset of types
> known by the server, perhaps using the following:
> 
> * /das/genome/type
> 
>   returns all types known to the server
> 
> * /das/genome/type?namespace=sofa.ontology
> 
>   returns all types known to the server under the "sofa.ontology"
>   namespace
> 
> * /das/genome/type?namespaces
> 
>   returns all type namespaces known to the server
>