map NPGObjExhText2.xlsx to TGN & AAT

[VladimirAlexiev]

NPGObjExhText2.xlsx includes useful data. american-art/npg#68 requests this data to be mapped (converted).
@si-npg It would be great if you map the used concepts:

• Execution place, Place depicted, Related place: to TGN
• Medium, Other Classification, Portrait Objects, Shape, Support: AAT
• Sitter Type is a special case. Eg "individual portrait", "group portrait", but I'm unsure about the other values
• Portrait Setting can be ignored, I think

Please also clarify the distinction between Place depicted, Related place in american-art/npg#68 (currently it says both should be mapped to Subject, i.e. depicted.