Had a bit of spare time so tried out the policy design pattern I've been learning about, applying it to PQ converters.

Some interesting results:
* Minor simplification of PQ converters and unit ctors; may not seem like much, but I think it enables possible future simplifications of a similar kind.
* CMeasurementPrefix is gone, no longer needed. Now prefixes are applied at the template stage.
* Apart from that, PQs public API is unchanged.
* Discovered that ft/s^2 conversion factor was wrong: fixed.

samples/blackmiscquantities/samplesphysicalquantities.cpp

@@ -15,15 +15,9 @@ namespace BlackMiscTest
  */
 int CSamplesPhysicalQuantities::samples()
 {
-
     // cases which must not work
-    // CMeasurementUnit mu; //must not work
     // CLengthUnit du1(CAngleUnit::rad());
 
-    CMeasurementPrefix pf1 = CMeasurementPrefix::h();
-    CMeasurementPrefix pf2 = CMeasurementPrefix::M();
-    qDebug() << pf1 << pf2 << (1.0 * pf1.toDouble());
-
     CLengthUnit lu1(CLengthUnit::cm());
     CLengthUnit lu2(CLengthUnit::ft());
     QString lu1s = lu1.toQString(true);