Create helper classes to iterate over points of an nD grid
Add a new CachedMeshGrid class to cache mesh points
Disable the cache, to not conflict with MaxNormMeshGrid.CachedMeshGrid
Also cache function evaluation
New self.dimension_ member to avoid calls to Distribution.getDimension()
Minor changes: Add a dot method, to avoid calls to ot.dot(), and multiply error by factor outside of the loop