d3viz improvements for Python 3

conda provides pydot-ng neither for 2.6 nor for 3.3

adding variable name in error messages raised by calling a function

Fix a test regression when we moved the new back-end

Gpu opt

Fix copy-paste error in docs.

More aliases, reordered.

Scan reintroduced benchmark

Make negative values for gpuarray.preallocate disable the allocation cache completely.