Coalesce data sources into one module

Kyo and I will be working on overhauling the way data loading is handled in the current RCMES workflow. Right now, the user manually specifies the sources for each dataset which are currently separated into three categories: local files on disk, the RCMES database (RCMED), and the Earth System Grid (ESGF). These cases are currently handled in separate modules / function calls, but it would be most ideal in the future to create one universal function call for all the data loading. An example schematic would be something like:

datasets = load(sources, ...)

Here datasets would be a list of OCW Dataset objects, sources would be a list of source specifications for each requested dataset (eg, 'esgf', 'local', or 'rcmed'). Ideally we would also like better support for handling datasets spanned by multiple files as well.