Fix rounding errors with fractional powers.
Simplify use of validate_power in Quantity.__array_prepare__ as
recommended by @mhvk
Add example about fractional powers.
Add CHANGES entry.
Convert floats into Fractions if they are close enough.
Disallow tuples as powers on Quantities
Fix test to test implicit float -> Fraction conversion.
Add more information in the documentation.
Fix doctest
Merge pull request #2070 from mdboom/unit/fractional-powers

Fix rounding errors with fractional powers.