Weights on examples

[afbarnard]

Add ability to handle weights on examples. This would necessitate a change to much of the current internals, basically changing integer counts to floating-point counts. However, integer and floating-point weights could be differentiated (e.g. as one of the type parameters) which may yield some benefits (accuracy) but would likely add complexity and necessitate changing the underlying arrays to collections. Anyway, this is a good design question.

Probably best to have tests of scale completed before working on this so we can compare the efficiency of an arrays-based design with the efficiency of a collections-based design.