Support preprocessing in to_units_container for string inputs

[eabruzzese]

Implement support for registry preprocessing against string inputs to the to_units_container util.

This allows string inputs to use the given registry preprocessors if available, e.g.:

from pint import UnitRegistry
from pint.utils import to_units_container

ureg = UnitRegistry(preprocessors=[lambda s: s.replace('%', ' percent ')])
ureg.define('percent = 0.01 = %')

# Used directly, specifying the registry with defined preprocessors.
to_units_container('%', registry=ureg)
# <UnitsContainer({'percent': 1})>

# Used via .to('%')
quantity = ureg.Quantity(1)
quantity.to('%')
# 100.0 <Unit('percent')>

• Closes AttributeError when using method "to" if a custom preprocessor is in place and a string is passed as target unit #1004 and AttributeError when evaluating "%" #1274
• Executed pre-commit run --all-files with no errors
• The change is fully covered by automated unit tests
• Documented in docs/ as appropriate
• Added an entry to the CHANGES file

[eabruzzese]

@hgrecco I think this closes/addresses several issues, though I'm not sure I've found all of them. I've added the ones I could find with my limited search vocabulary.

The unit tests pass and the change seems to work well for the use cases I've tried, but I'm not experienced enough with the library to know if my change will have any knock-on effects beyond what's covered by the test suite.

Do you think this is a usable solution, or am I misunderstanding something in #429?

[MichaelTiemannOSC]

Want!

[MichaelTiemannOSC]

xref #1687