Refactored Quantity.__cinit__() to simpflify arg list.  New Timings:

Unity          :   0.1033 s
Quantities     :    1.386 s
Pure Python    : 0.002114 s
Breakthough, using carrays instead of cpython.array.  Huge speedup:

Unity          :  0.02758 s
Quantities     :    1.403 s
Pure Python    :  0.00212 s
Fixed bug in __neg__.  Removed the memoryview access while I was there.
Commented code in assertQuantity that is not being used.
Cleaned out commented-out lines.
Added a global (module-level) switch to enable/disable unit baggage.  Does nothing yet.