* A constant library has been added with physical constants that can be accessed
by name or alias with the `constant` function. The `search_constants` function
with no argument gives a listing of all built-in constants. Each constant
definition includes any correlations with other constants.
* The `Quantity` class has been added to represent a general numerical value
multiplied by a unit and the `unit` function has been added to retrieve
`Unit` instances from the unit library by name or alias. `Unit` instances
can now be multiplied and divided by other `Unit` instances to produce
composite units, can be multiplied and divided by numbers to produce
`Quantity` instances or multiply or divide `Quantity` instances. The
`gummy` class is now a subclass of `Quantity` with a `nummy` value rather
than a subclass of `nummy`. A `QuantityArray` class has been introduced
to represent an array of values all with the same unit. Multiplying a `Unit`
instance by a list, tuple, or numpy array produces a `QuantityArray` instance.
* The `immy` class has been introduced as an `ummy` valued counterpart of the
`jummy` class for representing complex values with uncertainties. `immy`
and `jummy` values can now be displayed in a polar representation in addition
to a cartesian representation. `immy` and `jummy` .r and .phi properties
have been added to access the magnitude and argument of the values as a
complement to the .real and .imag properties.