value should reject non-numbers

[azaroth42]

e.g.

>>> from cromulent import model
>>> d = model.Dimension()
>>> d.value = "foo"
>>> print(model.factory.toString(d, compact=False))
{
  "@context": "https://linked.art/ns/v1/linked-art.json",
  "id": "http://lod.example.org/museum/Dimension/0",
  "type": "Dimension",
  "value": "foo"
}

This should fail to set d.value

[azaroth42]

Also content should reject non-strings, e.g. stmt.content={"foo"}