Supporting eager and lazy registration of providers #397
Description
With Phlex v0.1.0, data-product providers are eagerly 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 eagerly create providers for all data products within that file. Instead, there should be a mechanism that lazily registers providers based on the data products required by the data-flow graph.
There will likely be cases where both eagerly-created and lazily-created providers need to be used in the same job. We should discuss this.