v3.1.3

VortexStepMethod v3.1.3

Diff since v3.1.2

### Fixed
- bug in `linearize` where `body_aero.va` was used instead of `body_aero._va`,
  causing incorrect initial velocity storage (#227)
- `set_va!` no longer overwrites `_va` with a computed reference velocity when
  omega is nonzero; this simplifies the function and fixes linearization with
  turn rates
- linearize now correctly preserves and restores `omega` across perturbations

Merged pull requests:

• Fix bug in untested linearize branch (#227) (@1-Bart-1)