Temperature conversions are incorrect

[craigholm]

>>> from pint import UnitRegistry
>>> ureg = UnitRegistry()
>>> Q_ = ureg.Quantity
>>> Q_('0 * degK').to('degC')
<Quantity(0.0, 'degC')>
>>> Q_('0 * degC').to('degF')
<Quantity(0.0, 'degF')>
>>> Q_('100 * degC').to('degF')
<Quantity(55.55555555555556, 'degF')>
>>> Q_('212 * degF').to('degC')
<Quantity(381.6, 'degC')>

It seems like the "offset" parameter in the definition file is being ignored. The test_offset method in test_pint.py is decorated with unittest.expectedFailure.

[hgrecco]

The temperature conversion (and the support for logarithmic units like dB) is work in progress. The plan is to support two types of temperature units: difference in temperature and temperature. The difference in temperature will operate like any other multiplicative unit. I am looking for a way to implement all this without making the code too complicated. In particular, I would like to raise meaning full errors and warnings when an operation is not supported.

[craigholm]

If they don't work currently, it would be better to omit them from the definition file (at least on the release branch). Inaccurate results are worse than none.