Use VarAccess logic in ND4S

[riedelcastro]

VarAccess terms group a structured variable and an accessors to the variable. E.g. "theta.rels(2)" is an accessor of depth 2. These accessors make it easy to work with sparse gradients (they are generalizations of the lookup table). To convert a term tree to use these accessors, use Transformation.induceAccessors. Then create a VarAccessBox that efficiently deals with the sparse gradients.