Defer starting the SAS session until needed

[jsiirola]

Fixes # .

Summary/Motivation:

This resolves an issue with the SAS interface where the SAS session was being started in the interface __init__.py. This meant that any time the interface was created it immediately tried to connect - which, if you are offline, hung until the connection timed out (~10 seconds). With these changes (and some changes with how SAS tests are skipped), running the tests offline only hits the timeout once (and not a dozen times).

Changes proposed in this PR:

• Move the logic to start the SAS session into a helper method
• Only start the SAS session when needed (for the available() check or for solve())
• Update the SAS tests to minimize the number of times available() needed to be checked.

Legal Acknowledgement

By contributing to this software project, I have read the contribution guide and agree to the following terms and conditions for my contribution:

1. I agree my contributions are submitted under the BSD license.
2. I represent I am authorized to make the contributions and grant the license. If my employer has rights to intellectual property that includes these contributions, I represent that I have received permission to make contributions and grant the required license on behalf of that employer.