nasty bug in UOM.asPrimeMap. When adding two maps together it could
 lose data from the first map

Fixes #10
Merge pull request #11 from og3b/i10-qtybug

 #10 bug in multiplication of UOMs
RFC Position code

This is a stab at the position code. Any feedback welcome.

I need to at least clean it up and add more tests. Just want to make
sure it's going down the right track.