feat!: 💥 dimensionless quantities refactored

Dimensionless quantities are now represented by quantity types rather
than by plain representation types. Only dimensionless quantities with
`unitless` unit are implicitly convertible from representation types.

`units::exp()` now is a function doing std::exp() on a representation
type (previous `units::exp` class template was renamed to
`units::exponent`).

BREAKING_CHANGE: gcc-9.3 support removed
BREAKING_CHANGE: `exp` and `Exp` renamed to `exponent` and `Exponent`
Resolves #27
Resolves #42

test/unit_test/runtime/math_test.cpp

@@ -63,10 +63,12 @@ TEST_CASE("absolute functions on quantity returns the absolute value", "[math][a
       REQUIRE(abs(-1q_m) == 1q_m);
     }
 
+#ifndef COMP_MSVC
     SECTION ("floating-point representation")
     {
       REQUIRE(abs(-1.q_m) == 1q_m);
     }
+#endif
   }
 
   SECTION ("'abs()' on a positive quantity returns the abs")
@@ -76,10 +78,12 @@ TEST_CASE("absolute functions on quantity returns the absolute value", "[math][a
       REQUIRE(abs(1q_m) == 1q_m);
     }
 
+#ifndef COMP_MSVC
     SECTION ("floating-point representation")
     {
       REQUIRE(abs(1.q_m) == 1q_m);
     }
+#endif
   }
 }