Add attribute value parent for quantity value

[wdduncan]

In NMDC we have a general attribute value class from which quantity values, text values, images values, person values, etc. inherit. The attribute value class also has a was generated by for provenance info (more such slots can be added).

The advantage of this is that gives a kind of design pattern to add new value types, it is clear that it represents a value instead of a property/characteristic, and it allows annotate the value with metadata.

The NMDC quantity value class also has slots for:

• has raw value
• has minimum value
• has maximum value

These may be useful for biolink too.

cc @sierra-moxon @cmungall

[ramonawalls]

I support the use of a general attribute value class.