ObsCore section : dataproduct_subtype is not mandatory

[loumir]

proposal :
a- remove the first bullet on data product subtype from the list .

b- add a short caveat :
dataproduct_subtype is optional and free text.
it's content may vary between various experiments .

c- the serialization data format ( OGIP, VODF, etc ) is handled in Obscore in Access.format .
we must check that the related MIME types are allowed in ObsCore and if not add them .

[loumir]

the file format used in high energy data are not covered yet by Obscore 1.1
OGIP, VODF, etc should be added to the accepted format , or instead a vocabulary can be defined to identify all ObsCore accepted formats .

[mservillat]

I updated the text to resolve those issues, I hope this is fine

[mservillat]

commits where not properly integrated, now they are : #41