NIML hstack support + refactoring of Kohonen SOM

- add hstack support for NIML datasets; typical use is case is merging surface-based data from two hemispheres.
- refactor Kohonen SOM with separate _pretrain method; addresses http://lists.alioth.debian.org/pipermail/pkg-exppsy-pymvpa/2013q1/002480.html.
- support in Kohonen SOM for distance metric other than Euclidian.