Math functions not compatible with percent

[StefanoD]

Please include the following information in your issue:

1. Which version of units you are using
   Branch v3.x, Commit da6dd91

2. Which compiler exhibited the problem (including compiler version)
   GCC 11.4.0

The math function don't work for dimensionless units like percent

using units::concentration::percent;

percent<double> pct1 = 100_pct;
percent<double> pct2 = 70_pct;

percent<double> delta = fabs(pct1 - pct2); // Results into percent<double>(0.3)
percent<double> maxValue = fmax(pct1, pct2); // Results into percent<double>(1.0)
percent<double> minValue = fmin(pct1, pct2);  // Results into percent<double>(0.7)

// Following tests are failing:

EXPECT_EQ(delta, 30_pct);
EXPECT_EQ(maxValue, pct1);
EXPECT_EQ(minValue, pct2);

The problem lies in core.h where the said functions use .value() instead of .raw()

[chiphogg]

Seems very highly related to #276. Although I wouldn't go so far as to call this one a duplicate, I think some of the discussion there can be helpful.

Same for this comment on #275.