Allow newer C++ standards and fix missing headers

[yushill]

Describe the current behavior

The CMake configuration strictly enforces C++11 with CMAKE_CXX_STANDARD and CMAKE_CXX_STANDARD_REQUIRED TRUE.
Some source files rely on transitive includes (e.g. for <cmath> or <functional>), which may not be available with newer standard library versions.
A test also includes <numbers>, which is only available in C++20, causing issues when compiling with older standards.

Describe the expected behavior

• Allow newer C++ standards by unsetting CMAKE_CXX_STANDARD and CMAKE_CXX_STANDARD_REQUIRED
• Add explicit includes for <cmath> and <functional> where needed.
• Fix conditional inclusion of <numbers> to ensure compatibility with C++17 builds.

Describe the motivation

Improving compatibility with modern compilers and environments (e.g. C++17 or later).
Also prevents build errors caused by missing headers or incompatible standard versions.

Extra Information

Modification proposal:
main...yushill:powsybl-iidm4cpp:main