storage of task weights

Several interfaces have undergone some changes to support the storage of task weights.

Interfaces affected:
Store - new extend_Weights method, and the get_TaskSet and get_Task methods now support an optional list of weights to load.
Task - now has an associated dictionary of weightname-weight.
taskset_transform - When applying a transform, we first try to load any weights required from the store. After the transform has been applied, newly-computed weights are copied back into the store.
Transformer - transformers now all have a dictionary of weights associated with them. This interface doesn't feel very clean at the moment, and as such is likely to change soon.