Fix for https://github.com/Brewtarget/brewtarget/issues/634 plus more general tweak so we get an error message rather than an assert when trying to call toSI() for a BtLineEdit that does not related to a physical quantity. (Though we should fix the inheritance hierarchy for BtGenericEdit etc so that these things get flushed out at compile time.)
Fix for https://github.com/Brewtarget/brewtarget/issues/634 plus more general tweak so we get an error message rather than an assert when trying to call
toSI()for a BtLineEdit that does not related to a physical quantity. (Though we should fix the inheritance hierarchy for BtGenericEdit etc so that these things get flushed out at compile time.)