Fix math functions for percent

[StefanoD]

Fixes #328

Note: I tried to alter all math functions, but this resulted in some unit tests failing. There might be some rework needed from somebody with more knowledge.

[nholthaus]

I think this may stem from a misunderstanding of how concentrations are intended to work. The following tests will pass in the library as it currently exists:

percent resultPct = sqrt(16.0_pct);
EXPECT_EQ(resultPct, 4.0_pct);
EXPECT_EQ(0.04, resultPct);

So the idea is that 4% when it's a dimensionless absolute value is 0.04.

[chiphogg]

Shouldn't sqrt(16.0_pct) result in something like "4 perten" (made-up unit), which when converted to percent would result in 40.0_pct, not 4.0_pct?

[StefanoD]

If you go to my linked ticket, I showed different calculations which failed. This is IMHO a regression which previously worked in an older version.
Anyway: We are using the Aurora units lib now.