Code cleanup

[simogasp]

Hi, this PR cleans up a little bit the code with minor changes:

• constness for variables, function arguments, and methods
• avoid c-style casting but rather use static or reinterpret cast
• use range loops where possible
• remove virtualfrom overridden methods as it is not necessary
• use default for empty constructors/destructors
• use make_unique() instead of allocating explicitly with new
• define an implicit namespace detail instead of an anonymous namespace
• use empty() instead of checking size() == 0 for containers

I also had to raise the minimum CMake version because on recent versions the minimum that was required is not supported anymore.

I have some other code improvements requiring C++20 standard (concepts, ranges algo etc), if you are interested I can make another PR after this.
Also the CMakeLists for testing can be simplified quite a bit using fetch_content to retrieve the GTest automatically. I can make a PR about that too.