Merge branch 'master' into featuremapper

This commit merges the work on the featuremapper branch into master.
This work provides the basis for splitting the measurement code in
topographica out into a seperate module. It also changes the API
of the measurement code and the measurement storage containers. In
particular, the storage containers are now NdMappings, which allow
slicing syntax along their feature dimensions. The NdMapping objects
were designed together with Jean-Luc Stevens (jlstevens) and are
located in the imagen project.

Although every effort was made to allow loading of snapshots, there is
no guarantee that measurement defaults will be restored.
Merge branch 'master' into featuremapper