Add availability information for manuscript records

[wlpotter]

Since we have the list of which reading room each shelf mark can be called up in, we can add this and other information related to access restrictions on the manuscripts.

Best place in TEI would be:

msDesc/additional/adminInfo/availability. For multi-part, this should go at the top level. Cf. this TEI example and its section which uses child p elements to give info, so we could have one for which reading room to go to, maybe even info about photography permissions?

There is also an example of @status on the availability element, which could use the terminology from the BL related to "select", "restricted", etc.