Blender Git Loki

Redefinition of the meaning of "value type" for sockets (intermediate commit).

The use of "expressions" for node inputs must be generalized to support array-like
data and a level of functional programming. Inputs are now expressions *by default*.

A node can also have "variables" as inputs, which are implicitly defined in the base node
graph and should not be bounded in the first compiler step (i.e. are not defined by
pynodes). Variables instead are defined as outputs of other nodes (currently still called
"kernel" nodes), which would mostly constitute loops over such variables.

The concepts being worked on here are loosely based on those found in the Halide language
http://halide-lang.org/index.html