Provide better exposure for methods in pysat._files

[rstoneback]

Is your feature request related to a problem? Please describe.
Users may not be aware of built in file methods that could make their lives easier when adding new instruments.

Describe the solution you'd like
rename pysat._files.py to pysat.files.py

Describe alternatives you've considered

Additional context

[jklenzing]

I have a test branch up in #454. The change is straightforward and works, but rapidly becomes confusing to me due to the existence of the pysat.Files class and the methods under pysat.files, which also contains the Files class.

Possible routes:

• move the methods to pysat.utils.files and leave the main class file hidden.
• break these apart at the top level with a rename of one or the other

[rstoneback]

Moving the relevant methods to pysat.utils.files sounds like a good plan to me.

[jklenzing]

OK. I'll start mocking up a replacement PR. This will require deprecation warnings.

[jklenzing]

Looking over the code, it looks like from_os should really be moved from the class to utils.files as well. Thoughts?

[rstoneback]

That’s a class method. It has to be defined within the Files class.

[jklenzing]

Closing with merge of #505