factor out observatory and instrument model

[pahjbo]

The observatory / telescope and instrument parts of the model could become a useful source of information in themself - the instances would be slowly varying with time and could be managed within a git repository for instance to allow for easy collaborative editing. This is really instead of #3

[BaptisteCecconi]

In the Semantics WG, we are preparing the vocabulary for observation facilities (here it would be the observatory I guess). The semantic split between observatory and instrument is as follows:

• The observatory is the entity to which the location metadata (may be dynamic) is attached (i.e.: a telescope, a spacecraft), or grouping of observatories (e.g., the EHT)
• The instrument is the entity to which the instrumental capabilities metadata is attached (i.e., the spectral range, the field of view...)

I would be good that the definitions are clear in the document, and that could feed into the other document revisions (VOResource, ObsCore...) where the definition is really vague.