Add blade elements and buckling interface

[eirikurj]

Purpose

This PR addresses two things:

• adds bladeFSDT class
• adds KSBuckling interface

Originally developed by Tim (partially integrated by Ali/Nick). This PR is to clean up and splits into specific commits.

Before merging this please merge PR #3 first

Type of change

What types of change is it?
Select the appropriate type(s) that describe this PR

• New feature (non-breaking change which adds functionality)

Testing

Tests in pytacs repo can be run to test some of these additions. More tests should be added locally.

[A-CGray]

Before I approve this @eirikurj did you run the test case in pytacs that uses the bladeFSDT elements?
https://github.com/mdolab/pytacs/blob/master/tests/reg_tests/test_wing.py

[eirikurj]

Yes, I did update the tests and added a new example.
There is a pending PR here, https://github.com/mdolab/pytacs/pull/27

[A-CGray]

Oh perfect, nice one!