Convert to inverse unit incorrect

[Gra-tak]

Thanks for this awesome library. As far as I can tell, I have found an error in the conversion between inverse magnitudes like Hertz and seconds.

units::precise_unit unitA=units::unit_from_string("ms");
units::precise_unit unitB=units::unit_from_string("kHz");
double v=units::convert(1, unitA, unitB);
double v2=units::convert(v, unitB, unitA);
std::cout << "v = '" << v << "'" << std::endl;
std::cout << "v2 = '" << v2 << "'" << std::endl;

gives the following output:

v = '1e+06'
v2 = '1e-12'

As far as I can tell,
return result.multiplier() / (val * start.multiplier());
should be replaced by
1.0 / (val * start.multiplier() * result.multiplier());.

[phlptp]

You are correct. I had never tested that code path with both units having multipliers.
Added some tests and fixed the issue in #166

Thanks for catching that.