v0.7.1

Added

• Accept a single exact spelling string for prefixable, equivalent to a
  one-element tuple, with the same spelling validation before registration.
• Expand the si namespace's prefixed shortcuts and common units, including
  percent, permille, permyriad, rotational frequency (rpm), speed of light
  (c), gram-force, volt-ampere, ampere-hour, mho, and nit. Enable Torr prefixes.
• Add SI.kappa and the wavelength and frequency Wien displacement constants,
  SI.b and SI.b_.

Changed

• Use fixed Decimal approximations with about 50 significant digits for pi and
  the positive roots in the Wien displacement laws. Derived constants still use
  the Decimal context active at import.
• Treat u and Da as spellings of one formal unit, with prefixes enabled only
  on Da. Rebuild stored units when migrating from the separate u identity;
  the mass conversion scale is unchanged. Use kDa rather than kilodalton.
• Keep ASCII micro spellings such as um and us convenient to type without
  warnings. The explicit product u m retains its separate mass-times-length
  meaning.

Full changelog: v0.7.0...v0.7.1