Openaerostruct

This is a patch release for OpenAeroStruct. This contains fixes to make the load transfer work-consistent as originally described in the OAS journal paper. For reasonably fine meshes, the differences in results relative to the previous release will be very small.

See Issue 314 and Pull Request 318 for details.

This is a minor release for OpenAeroStruct. Highlights include:
- Updated all code for Python3 and OpenMDAO 3.0+ support only.
- Added efficient sparse analytic derivatives for most components.
- Expanded documentation and examples for wingbox structure.
- Added Prandtl-Glauert correction, sideslip angle, point mass and thrust capabilities,
- Added black-box example useful for users who don't want to interface with OpenMDAO directly.

This release focused on decreasing installation barriers and increasing computational speed. We also expanded the breadth of testing and introduced some more examples.

* Removed all Fortran code
* Introduced sparse analytic Python-based derivatives for most all components
* Formalized more of the wingbox structural model, though it still is not differentiated
* Increased test coverage to >95% of the codebase

This is the first official release of OpenAeroStruct to use the newly developed OpenMDAO 2.3+.

We've increased code modularity with less behind-the-scenes magic to expose the user to more of the OpenMDAO functionality.
This expands the capability of OpenAeroStruct to more easily allow multipoint optimizations and interfacing with other codes written in OpenMDAO.