Avoid unnecessary parentheses for Units with complex value when using `format`

Describe the bug Some units are considered complex units, meaning they are treated differently to normal, "numeric" units. This for example is the case for the volt-ampere reactive, considered to be VAR by mathjs.

To Reproduce I am unsure why, but converting these units leads to unexpected results:

(new Unit(1, 'MW')).to('kW').format() // 1000 kW - correct
(new Unit(1, 'MVAR')).to('kVAR').format() // (1000) kVAR - weird
(new Unit(1, 'MVAR')).to('kW').format() // (1000i) kW - weird

I understand the want to represent these as complex numbers, but I think this is fairly out of place when dealing with units. It leads to unexpected conversions and format outputs.

Notice that using formatUnits avoids this, of course, since only the number part is affected by this:

(new Unit(1, 'MVAR')).to('kVAR').formatUnits() // kVAR
(new Unit(1, 'MVAR')).to('kW').formatUnits() // kW