Approximate linear lift curve slope with polar corrections

[ngoiz]

Supports corrections from polar airfoil data on the linearised aeroelastic system.

Modifications are only made to the lift force at the moment. The forces from the aeroelastic system are projected onto the local stability axis (drag, lift, etc) and a coefficient is applied based on the derivative of the polar data at the linearisation condition. The forces are then projected back onto the original frame.

Full details found in the docstring: https://github.com/ImperialCollegeLondon/sharpy/blob/eceff9c70d77e100c358b1b9309eb88363cdebee/sharpy/generators/polaraeroforces.py#L219

@rafapalacios @ACea15 please can you have a look whenever you have a moment? maybe we don't want to merge this just yet or maybe you think its a valuable feature

[ngoiz]

Thanks, Alvaro, for the review. I'm seeing if can fix an issue with the tests (running fine from the individual directory but failing from the top-level) and I'll merge this