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convert between different measurement units

Project Description
What is firkin?

firkin is a python module to convert between different measurement


First we create an instance of `UnitManager`::

>>> um=UnitManager()

Next we create two families of units. The first one ist ``liter`` and
uses ``SIFamily`` to automatically create units with the SI prefixes::

>>> um.add(SIFamily(base='l', name='liter'))

Now our ``UnitManager`` knows about fl, pl, nl, ..., Ml, Gl, Tl.

How many liters are 10000 ml?::

>>> um.convert_to_unit(1e4, 'ml', 'l')
(Decimal("10.0000"), 'l')

Next we create a family by hand::

>>> f=Family(name='f', base='gallon')
>>> f.add('barrel', 36, 'gallon')
>>> f.add('kilderkin', 0.5, 'barrel')
>>> f.add('firkin', 0.5, 'kilderkin')

Now we have a family called ``f`` that used gallon as its base and knows about
barrel, kilderkin and firkin, too.

How much gallons is one firkin?::

>>> f.convert(1, 'firkin', 'gallon')
(Decimal("9.00"), 'gallon')

What's the best way to express 3 kilderkin?::

>>> f.autoconvert(3, 'kilderkin')
(Decimal("1.50"), 'barrel')

To convert between family ``f`` and family ``liter`` we need to add ``f`` to
our ``UnitManager`` and tell how much liters (base unit of family ``liter``) a
gallon (base unit of family ``f``) is::

>>> um.add(f, other='liter', factor=4.54609)

Of course the ``UnitManger`` can convert firkin to gallon, too::

>>> um.convert_to_unit(1, 'firkin', 'gallon')
(Decimal("9.00"), 'gallon')

But it also can convert firkin to liters::

>>> um.convert_to_unit(1, 'firkin', 'l')
(Decimal("40.9148100"), 'l')

Or find the best way to express one liter in one of the units from
family ``f``::

>>> um.convert_to_family(1, 'l', 'f')
(Decimal("0.219969248299"), 'gallon')

That works with barrels, too::

>>> um.convert_to_family(1, 'barrel', 'f')
(Decimal("1.00"), 'barrel')
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