Implement Quantity.to_no_prefix for stripping SI-prefix

[nikosavola]

• 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

This PR adds a method for Quantity to return a copy with no SI-prefixes, e.g., nanometers to meters w/o specifying that meters are the desired unit.

import pint
Q_ = pint.Quantity
(Q_('8 um')).to_no_prefix()

returns

<Quantity(8e-6, 'meter')>

[hgrecco]

I think the implementation still needs work. Converting to a string and then replacing by "" is fragile. It would be better to iterate over units, parse each individually and then remove the prefix. Take a look at the Unit api.

[jules-ch]

I agree with @hgrecco, there is already a function that provides parsing of prefix and units. I'll provide a better review soon.
Difficult for me to provide a review at the moment.