Use base SI unit prefixes in custom Quantities when no custom prefixes are provided

[shsms]

Using standard prefixes should be enough for quantities like kW, kHz, etc. It should still be possible to specify exponent prefix maps, for cases like sq.km, which is 10e6 sq.m.

Originally posted by @llucax in frequenz-floss/frequenz-sdk-python#422 (comment)