Update of requirements draft

Hi all,

Just to say thanks for all your contributions this week :) , and let you
know what has happend to the requirements document since the f2f meeting:

- Updated introduction with material from Joakim - I used not everything
from him, so there still might be some changes here.

- Created a section "Purpose of the Ontology and the API" that contains
Tobias' figure and an explanation for it.

- Added the material from Jean-Pierre about the "Recommendation across
different media types " use case.

- Updated "cultural heritage use case" material

- Created a new requirement "Provide support for controlled vocabulary
for the values of different properties", which is related to the
"cultural heritage" uc

- Added the issue of return values to the requirements section. Btw.,
looking into XMP again, I noticed that we have also only singular
(return ) value types here, e.g. for dc:subject a "bag of Text".

What still needs to be done:

- Add (e.g. for MMSXG deliverables) & edit references

- Send this to the fragments WG. I will do that tomorrow (Saturday)
morning my (Japanese) time. I will try to integrate as much as possible
of the feedback I get before.

Best,

Felix