convert a value from one set of units to another

def convert(self, value, fromunits, tounits):
        '''convert a value from one set of units to another'''
        if fromunits == tounits:
            return value
        if (fromunits,tounits) in self.unitmap:
            return value * self.unitmap[(fromunits,tounits)]
        if (tounits,fromunits) in self.unitmap:
            return value / self.unitmap[(tounits,fromunits)]
        raise fgFDMError("unknown unit mapping (%s,%s)" % (fromunits, tounits))