Support user-specified map/dict of directory structure.

[spencerahill]

The GFDL directory structure logic I use is specific to my own naming conventions, i.e. atmos_level_inst for instantaneous atmospheric data on model-native coordinates. However, not all people use these same names -- they can be named basically arbitrarily in the simulation's XML. Therefore, the logic for traversing a GFDL directory of model output should support user-supplied names.

The good thing is that, besides the names of the components, the structure is pretty much locked in, meaning that only the strings would have to be modified. The specification of these names could potentially vary from run to run within a model, and so it should be supported to be specified at the Run, Model, or Proj level. E.g. `component_names={'p.inst': 'atmos_inst', 'eta.inst': 'atmos_level_inst'}. This needs to be thought out further.

[spencerahill]

This hasn't been accomplished yet, but it has been subsumed by the larger revamp of the file-finding methods addressed by #32 so no need for its own open issue.