Definition of graph templates

[mariomac]

In some cases, users should't need to define the whole graph structure but maybe just a subset. For example, in a monitoring agent, they could define the samplers + the submitters but they shouldn't need to define some internal processing.

We need a way to provide a predefined configuration structure with some empty gaps that could be defined by the user, this would include:

• 1:1 gap. The user defines a single node that sends data to another node.
• n:1 gap. The user can define multiple parallel nodes that send data to another node.
• 1:n gap. The user can define a single node that sends data to other multiple nodes (already defined in the template)
• n:n gap.
• defined node group. The user can fill the gap with its own node group. Required: https://github.com/mariomac/pipes/issues/3
• predefined node group. The user can fill the gap just putting there a pre-defined node group.

[mariomac]

TODO: Reason whether this is out of the scope of this library, as this might be handled by a configuration library.