ENH: Default pysat directory no longer specified.

[rstoneback]

Description

Addresses

pysat no longer sets a default directory for data storage.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Travis CI

Checklist:

• Make sure you are merging into the develop (not master) branch
• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• Add a note to CHANGELOG.md, summarizing the changes

[rstoneback]

Last pull pre 2.1.0

[jklenzing]

Thanks @rstoneback!

[jklenzing]

Does this close #315, or is there something else we need there?

[rstoneback]

Added a runtime check for data_dir and created a unit test for the same situation.

[rstoneback]

I believe this now closes #315. If a user can't create a pysat.Instrument object without a data_dir and we don't ever set one for them, we should be good.