Adding a simple nacelle drag model in AeroDyn

[mayankchetan]

NOT ready to be merged

Feature or improvement description

Added a static drag model to account for drag on the nacelle.

Related issue, if one exists

#162

Impacted areas of the software

AeroDyn

Additional supporting information

• AeroDyn input file updated to include, nacelle projected area, Cd along projected area, and the aerodynamic center where the nacelle drag force is applied.

Test results, if applicable

TODO

• [ ] Update api_change.rst for API changes
• [ ] Update test files to account for API changes
• [ ] Add test for the nacelle drag model
• [ ] Document implementation

attn: @ebranlard, @gdeskos