intersection gimmicks into their own functions with their own test suite.

This turned up two bugs:

1. The mass weighted union gimmick was incorrect when passed a list with
   a single mapping.  In that case, it neglected to multiply the mapping
   by the given weight.

2. The underlying weighted{Intersection, Union} code does something crazy
   if you pass it weights less than 0.  I had vaguely hoped to be able
   to subtract scores by passing 1 and -1 as weights, but this doesn't
   work.  It's hard to say exactly what it does then.  The line
       weightedUnion(IIBTree(), mapping, 1, -2)
   seems to return a mapping with the same keys, but *all* of whose
   values are -2, regardless of the original mapping's values.