Implement datachannel

[dumoulin]

Remove the global variable system and replace it by capsule to capsule data channels.

Actually, we can specify the variables needed as input and produced as output for a processor. And this specifications are checked through the workflow from a processor with its successor.

But, this specifications aren't more propagated. For example, let's consider 3 processors P1, P2 and P3, and these specifications :

P1: input=v1 ; output=v2, v3
P2: input=v2; output=v5
P3: input=v3, v4; output=v6

The variable v3 needed by P3 will not be found because she is not declared by P2, although this is P1 that produces it. The variable v4 is needed by P3 but isn't produced by any processor. So this variable is needed at the start of the workflow (as v1).

Datachannel will address this problem !