Changes all occurrences of the 'env' attribute to 'fgraph', and
renames Env to FunctionGraph. Adds properties that warn about
deprecated 'env' attribute.

Fix conv subsampling

When using "valid" convolutions with subsampling, the gradient
implementation was wrong (if dx or dy > 2) or slow.
In that case, we now use the implementation in conv3D instead.

Misc for IPAM GSS 2012

Rename env

added some documentation to gof