[QUANTITY] Use-cases, role in larger scheme

[Jonathan McDowell]

David,
 I understand your argument. However, it is clear from the October
email discussion that a QuantityWithFrameAndMappings is too heavy
an object to gain acceptance right now with a signficant fraction
of the participants. I therefore believe we should define a
QuantityWithUnitsAndMaybeFrameButNotMappings as our first pass on
an agreement. There is nothing to stop us continuing to elaborate
a QWithMappings object that would be derived from it, and when
agreement is reached on such an object the simpler object becomes
a special case. I think it's separable enough that we won't have
to go back and break the simpler object once we agree on the mappings.
But for now, I am going to ask that we make the cut for our first
version of the object by leaving Mappings out. This is the only way
for us to get an agreed document in a short time.
 Jonathan