Add Marker class - an object that identifies a person buried in a grave

[Stephen-Gates]

Following on from an email conversation with Rob Warren...

Me:What I didn’t find was the concept of markers. These are labelled metal stakes that represent a person buried in a grave. Is that catered for?

Rob: Good point. Please create a github ticket requesting a "Marker" or "Stake" class. In the current ontology, the top-level class for this is a "Monument". Do you think that "CommemorationObject" would be a better superclass or is a metal stakes a Monument?

In some cemeteries a marker is used to identify each person buried in a grave. Markers are sometimes a stake pushed into the ground beside the grave, or sometimes embedded into the grave surrounds.

In my experience, a Marker identifies one, and only one, person's remains in a grave - there may be different practices followed elsewhere.

I'll open a separate issue regarding the "Monument" or "CommemorationObject" superclass question.

[rwarren2]

In the ontology, a Monument can identify multiple graves and / or persons. I'm going to create Marker the same way.

Working definition: "A marker is a stake, flag, sign or peg indicating the location of a grave"