Merge: StatisticMap and statisticType attribute instead of (T|F|Z)StatisticMap [ following the Hackathon ]

As discussed at the hackathon/HBM 2014 with @chrisfilo, @nicholst, @nicholsn, @khelm, @jbpoline, @gllmflndn and Tibor Auer (see also #68). 

This is a proposal to replace `TStatisticMap`/`FStatisticMap`/`ZStatisticMap` types by `StatisticMap` with a `statisticType` attribute. This will, in particular, ease querying across different statistic type.