Supporting explicit and implicit registration of providers

[knoepfel]

With Phlex v0.1.0, data-product providers are explicitly registered based on which sources are loaded. Some providers may not be used if there is no downstream algorithm that requires the data products from that provider.

This approach is not sufficient. When reading framework files (e.g., as produced by FORM), it is impractical to explicitly create providers for all data products within that file. Instead, there should be a mechanism that implicitly registers providers based on the data products required by the data-flow graph.

There will likely be cases where both explicitly-created and implicitly-created providers need to be used in the same job. We should discuss this.

[knoepfel]

Agreement:

1. Providers attached to an input source will be automatically registered for any data products that cannot be provided by an explicit provider or a computational node.
2. We need to allow users to indicate a precedence of looking up data products from different stages.