Negating Gains

Is it intentional that this works (`-2u"dB"`), and this works (`0u"dB" - 2u"dB"`) but not this (`-(2u"dB")`) nor (`-uconvertp(u"dB", 2)`) nor this (`-uconvertrp(u"dB", 2)`)?

No source file or test is named. Start by reproducing the three negation cases in Julia and trace how unary negation interacts with unitful quantities and conversions. Done means either making the unsupported forms behave consistently with the working forms, with regression coverage, or documenting why the difference is intentional.

Written by the indexing model from the issue text.