Aerosandbox

- Added first experimental ability to support black-box functions in optimization, available in `aerosandbox.modeling.black_box`. This is a very early prototype, and the API is subject to change.
- Fixed a bug in `asb.Airplane` auto-generation of s_ref, c_ref, and b_ref that would cause hard-coded s_ref values in the constructor to not be applied to the Airplane instance. Also, adds the capability for auto-generation of reference values from fuselages, if no wing is present.
- Fixed a bug within `asb.AeroBuildup` effective span calculation for symmetric wings that were used to represent doubled vertical stabilizers. Now, if you create an aircraft with twin vertical stabilizers, the Trefftz-plane wake will correctly not carry over between the two vstabs. (If you have a symmetric horizontal stabilizer, it will still carry over - currently this is a function of effective dihedral angle.)
- Fixed Torenbeek wing weight model to use a simpler model for speedbrake / spoiler weight. This resolves a (possible?) error in the wing weight model due to an ambiguity in the source text (Torenbeek's "Synthesis of Subsonic Airplane Design", Appendix C) about what area they were referring to.
- Fixed a bug with uncertainty bootstrapping in `aerosandbox.tools.pretty_plots`, when normalizing input data.

- BREAKING: Critical bugfix in `asb.OperatingPoint.compute_rotation_matrix_wind_to_geometry()`, which in turn affects `asb.OperatingPoint.compute_freestream_direction_geometry_axes()`: A sign was reversed in the rotation matrix calculation for the sideslip angle beta. This resulted in the sign convention for beta (sideslip) being flipped, ultimately causing a sign error. In practice, this bug affected `asb.VortexLatticeMethod` analyses with nonzero beta, causing flipped signs for `CY, Cn`, etc. This bug was introduced in AeroSandbox v4.0.0, and was present in all versions of AeroSandbox v4.0.0-v4.0.6. This bug did not affect `asb.AeroBuildup` or any other aerodynamics analysis methods. (Thanks to carlitador for catching this.)
- Added `asb.AVL.open_interactive()`, to interactively launch an AVL session.
- Improved `__repr__` methods throughout for better readability.
- Updated `asb.AeroBuildup` to add induced drag on a whole-airplane level, not per-lifting-object. In general, this will result in slightly higher induced drag, and also improves optimization pressure - for example, tandem-wing configurations are no longer unrealistically attractive, since the induced drag scales superlinearly with respect to total lift.
- Improved `asb.Airfoil.generate_polars()` to not error out when a cache filename is provided where the containing directory does not yet exist. (Instead, it now creates the directory.) Fixed a bug in `generate_polars()` that now allows any Reynolds number input list to be specified when calling XFoil.
- Added `asb.Airfoil.plot_polars()` to make polar functions more interpretable.
- Added an optional `color` attribute to `Wing`, `Fuselage`, and `Propulsor` objects, which can be set during instantiation or manually set afterwards. This will control the color that the component is drawn with during visualization; currently only works with the `Airplane.draw_wireframe()` and `Airplane.draw_three_view()` methods, but will be extended to others later.
- Updated pinned CasADi version to 3.6.1. Note that CasADi MUST be v3.6.0 or higher, otherwise automatic differentiation errors may occur due to undefined primitives. This shouldn't be an issue, as this is now pinned in ASB's setup.py.

- Better documentation. Added a hotfix to support CasADi 3.6.0, which made a breaking change by removing `casadi.mod`.

- Better documentation only, improved README, etc.

- Baseline version for changelog (started tracking here)

- [Online-hosted documentation](https://aerosandbox.readthedocs.io/en/master/) set up