Make Variables an independent class

[rwegener2]

Goal

Right now the variables class assumes that it is either being initialized using a file or an order. This Issue proposes that an instance of Variables should be a stand-alone class which can be invoked with as little as a product string.

Motivation

1. The ability to generate a Variables list is a unique feature of icepyx as compared to other IS2 reading modules. Making it stand alone would make that feature more flexible, and we hope more widely used
2. Doing this will allow us to more easily integrate the cloud reading feature, since it broadens the class to accept more inputs than just "order" or "file"

Use cases:

• user wants to use Variables with just a product / version. Their read method is likely something else.
• Internally, allow the Query or Read modules to access the available variables.

Todo

• add input argument that is just user defined strings for product and version (default to "latest" version?). Main task of init is to define these.
• set to read vars by default from prod/version, and read from path only if there is one

To really promote this change, it will be important to address it both in the context of the code and the documentation. More discussion can be had on how exactly to update the documentation to make it clear that Variables can be used in the context of a Query/Read or independently.

[JessicaS11]

update the documentation to make it clear that Variables can be used in the context of a Query/Read or independently.

This issue is largely complete, but I'm leaving it open to bookmark expanding the docs (beyond the notes in examples) to reflect this use pattern.