Need a JGF reader to populate the resource graph data store

[dongahn]

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[dongahn]

Just had a discussion with @milroy and @SteVwonder:

The current plan is to rename generators into readers and refactor the current flat class structure into a two-level hierarchy -- resource_reader_base_t as the base class and resource_reader_grug_t, resource_reader_hwloc_t, and resource_reader_jgf_t as derived classes. I will also use a factory class pattern to help the upper layer written with the base reader class as much as possible.

Then, I will add one method called load into resource_graph_db_t, which takes in a base reader object and a serialized string as input and newly populates its resource graph.

Expanding this, @milroy will further extend resource_graph_db_t with additional grow and shrink methods in support of #511 and later elastic scheduling.

[dongahn]

resource_graph_db_t will be carved out from resource/schema subdirectory to its own subdirectory called resource/store. Two advantages: 1) We don't want to create an additional dependency into schema -- in fact it will create a circular dependency between schema and readers; 2) as resource_graph_db_t will be extended in the future for elastic scheduling support, we don't want to keep this as a headerfile only implementation, but rather treat this as a first class citizen: its own header and implementation files.