Implement `minVal` and `maxVal` without references to be usable by static constexpr members

## Brief feature description

Currently even constexpr static members in C++ must be explicitly instantiated in a cpp file. With C++17 and inline constexpr this issue is fixed.
If one would like to use a constexpr static member in C++ in combination with the constexpr `algorithm::minVal/maxVal` one gets a linker error since it is not explicitly instantiated and those methods require a reference.

This could be solved by adding a `constexpr` version of minVal/maxVal which are using non - references as parameters.

See this code snippet to illustrate the problem:
```
// in some header
struct Bla {
static constexpr int SomeVal = 123;
};

// in another header we use
// constexpr T minVal(const T &a, const T &b);
cxx::vector blubb; // leads to linker error

// fix:
constexpr T minVal(const T a, const T b);
```

Locate algorithm::minVal and maxVal and their existing tests; first inspect the reference-taking overloads and run the focused algorithm test target. Done means the constexpr static-member example compiles without a linker error and the new behavior is covered by tests.

Written by the indexing model from the issue text.