Add check for delta unit to convert

[dalito]

• Closes pint is converting delta_degree_Celsius to degree_Fahrenheit #1904 (reported in a discussion not as issue)
• Executed pre-commit run --all-files with no errors
• The change is fully covered by automated unit tests
• Added an entry to the CHANGES file

This fixes an oversight in offset unit handling. Offset units have a "delta" counterpart. Converting between offset unit and its delta counterpart is not possible. This was not checked until now resulting in e.g.

>>> import pint
>>> ureg = pint.UnitRegistry()
>>> ureg.Quantity(10, ureg.delta_degC).to("degC")
<Quantity(-263.15, 'degree_Celsius')>

With the PR a DimensionalityError is raised:

>>> ureg.Quantity(10, ureg.delta_degC).to("degC")
...
pint.errors.DimensionalityError: Cannot convert from 'delta_degree_Celsius' to 'degree_Celsius'

Closes #1906

[codspeed-hq]

CodSpeed Performance Report

Merging #1905 will not alter performance

_{Comparing dalito:issue1904-delta-bug (991b259) with master (5206fac)}

Summary

✅ 437 untouched benchmarks