handling multiple files with different datasets/variables

[JessicaS11]

Stemming from some of the discussion in #72, a key consideration is what will an icepyx.data object hold and what operations can be done on those files. A few key questions:

1. can a single data object "contain"/handle files for multiple datasets?
2. Can a single data object hold files that contain different sets of variables (even if they're all the same dataset)?
3. What information, if any, do we expect the object to be able to provide about those datasets, particularly if it was not created from an icepyx.query object (i.e. will it in any way iterate through the data files (names or metadata) and provide summary spatial, temporal, etc. information)?

All of these will need to be answered before we can fully implement any sort of data management and manipulation scheme, including more advanced filtering, input/output, and file reading.