'Create a new UnitDbl object.
Units are internally converted to km, rad, and sec.  The only
valid inputs for units are [ m, km, mile, rad, deg, sec, min, hour ].
The field UnitDbl.value will contain the converted value.  Use
the convert() method to get a specific type of units back.
= ERROR CONDITIONS
- If the input units are not in the allowed list, an error is thrown.
= INPUT VARIABLES
- value    The numeric value of the UnitDbl.
- units    The string name of the units the value is in.'

def __init__(self, value, units):

    self.checkUnits(units)
    data = self.allowed[units]
    self._value = float((value * data[0]))
    self._units = data[1]
