v0.28.0

This request includes a number of trigonometric improvements as well as new units for Time. Many thanks to adamreichold and Aehmlo for pull requests included and issues resolved in this release.

Added

• #182 Add inverse trigonometric functions to Ratio (acos, acosh, asin, asinh, atan, atanh) and Angle (atan2).
• #184 Tropical and sidereal units added to Time.

Changed

• #186 Make hypot available for all quantities, not just Length.
• #187 [Breaking] Change trigonometric functions to return Ratio instead of the underlying storage type so that identities like x.sin().asin() == x are well-typed.