Improved precision of PoundForce unit (thanks Jim Selikoff).

Added angle units of measurement (thanks Georgios).
Added tests and fixed bug in NewtonPerSquareCentimeter and NewtonPerSquareMillimeter.

Added imperial and US units for volume and area.

Added metric area units.
Fixed bug where exception was thrown instead of returning false for TryConvert to volume units.

Added metric volume units.
Fixed bug where some TryConvert cases threw exception instead of returning false.