Implementend US and imperial volume units with unit tests:
CubicMiles, CubicYards, CubicFeet, CubicInches, UsGallons, UsOunces, ImperialGallons, ImperialOunces.
Added missing tests for imperial area units in UnitConverterTests.cs.
Bumped to 1.7.0.

<releaseNotes>Added imperial and US units for volume and area.</releaseNotes>