Corrections for 3.2.0 release (jjonkman review)

[andrew-platt]

This is ready to merge.

This PR contains a few requested corrections for the 3.2.0 release in review #1175 (review)

Corrections include

• Correction to angle perturbation for TStC and SStC to use small angle (this now matches the implementation plan for linearization)
• Correction to angle perturbation for tower in AD15 to use small angle (this now matches the implementation plan for linearization)
• Typo in api_change.rst
• relocate TurbSim entry in api_change.rst
• removal of incorrect line of text in FAST.Farm documentation in fast.farm/index.rst

Tests
I don't believe any regression test results will be affected.

The change in the angular perturbations to use small angle assumptions instead of large angles has a very small effect in the resulting DCM (small angle method is slightly more accurate near 0, so we use it when we know angles remain small). So the DCM produced using the small angle method will be very slightly different than the large angle forumaltion, but the difference is very small. The effect of this on the linearization of the AD15 tower becomes negligible since the angular perturbation of tower nodes in AD15 has little effect in the aero loads. Similarly the structural controls (StCs) are not very sensitive to small angular changes, so the minor difference in the DCM will have no impact on the linearization results.

[jjonkman]

I agree that the effect will be small. I just want to be consistent in how the meshes that get mapped together treat 3D orientations in the linearization. Thanks for addressing my comments.