add ZipForeachValues (but not working)
zipValues, which is designed to allow traversing over a pair of
tensor's values.
simple distance implementation based on zipValues
fix to caching for SparseVector
Merge branch 'dist' of https://github.com/dlwh/breeze into dlwh-dist
fix the return type to Unit in foreach
change distance to squaredDistance
add a specialized implementation for zipValues[DenseVector, DenseVector], which brings
back the performance. The generic one needs multiplication to locate elements.
Merge pull request #162 from mengxr/dist

Add zipValues and squaredDistance for vectors