Compiler type --> CTS type
More checks to DoublePrefixedUnitConversionProvider
Added conversion infra for IMeasurable<TValue>

- Added DoubleMeasurable (implementation of IMeasurable<TValue> for
double, inherits from MeasurableBase<double>);
- Added base class for measurable unit conversion providers and its
child for double;
- Added corresponding unit tests;
- Updated Units sample with IMeasurable<TValue> and its conversions
usage.