1.2.1

Fixed

• The core builds on Linux. UnitAngularVelocity, UnitDensity, UnitMassFlowRate and
  UnitVolumetricFlowRate provide the init(symbol:) that swift-corelibs-foundation requires of
  every Dimension subclass, and String(localized:) resolves to its English key there, as
  open-source Foundation has no string catalogs. CI now builds and tests the core in a Linux
  container.