def get(self, varname, idx=0, units=None):
        '''get a variable value'''
        if not varname in self.mapping.vars:
            raise fgFDMError('Unknown variable %s' % varname)
        if idx >= self.mapping.vars[varname].arraylength:
            raise fgFDMError('index of %s beyond end of array idx=%u arraylength=%u' % (
                varname, idx, self.mapping.vars[varname].arraylength))
        value = self.values[self.mapping.vars[varname].index + idx]
        if units:
            value = self.convert(value, self.mapping.vars[varname].units, units)
        return value
get a variable value