ContrastWeights and statisticType attribute instead of (T|F)ContrastWeights [ following the Hackathon ]

As discussed at the hackathon/HBM 2014 (see also #68). 

This is a proposal to replace `TContrastWeights`/`FContrastWeights`  types by `ContrastWeights`with a `statisticType` attribute specifying. This will, in particular, ease querying across different inference activities.

This is similar to what was done for `StatisticMap` and `Inference` in Pull Request #80 and #81.