constexpr LEVELS for easier custom level usage

[SzGaa]

Usage of C++20's new constexpr functionalities for easier usage of custom LEVELS!

Modified:

• With C++20 LEVELS's constructor can have a constexpr specifier
• const LEVELS can be replaced to be constexpr's instead

PULL REQUEST DESCRIPTION

Easy, but not too readable solution.

#if __cplusplus >= 202002L

This functionality provides an easier usage of custom log levels as value can be used within switch statements.
No explicit constant is required to store & work with custom LEVEL's value.

		//  TRACE is between DBUG and INFO
	constexpr LEVELS TRACE { (DBUG.value + INFO.value) / 2, "TRACE"};

	void ReceiveLogMessage( g3::LogMessageMover logEntry)
	{
		switch ( logEntry.get()._level)
		{
			case my_namespace::TRACE.value:
			...
			case g3::kWarningValue:
			...
		}

Warning:
On the example above 'DBUG' is used as 'CHANGE_G3LOG_DEBUG_TO_DBUG' cmake option was set.

Testing

• This new/modified code was covered by unit tests.

• (insight) Was all tests written using TDD (Test Driven Development) style?

• The CI (Windows, Linux, OSX) are working without issues.

• Was new functionality documented?

• The testing steps 1 - 2 below were followed

step 1

mkdir build; cd build; cmake -DADD_G3LOG_UNIT_TEST=ON ..

// linux/osx alternative, simply run: ./scripts/buildAndRunTests.sh

step 2: use one of these alternatives to run tests:

• Cross-Platform: ctest
• or ctest -V for verbose output
• Linux: make test

[KjellKod]

Very cool.

Could you add something to DOC: g3log-with-sinks that shows how the usage would improve with C++20?

I.e. the current example shows this

  FG_Color GetColor(const LEVELS level) const {
     if (level.value == WARNING.value) { return YELLOW; }
     if (level.value == DEBUG.value) { return GREEN; }
     if (g3::internal::wasFatal(level)) { return RED; }

     return WHITE;
  }

[KjellKod]

and maybe to here also? DOC: custom logging levels

[SzGaa]

Yes, I'll have a look on the docs too.

[KjellKod]

@SzGaa I am planning to do a minor release update when the next change comes in. It would be great to get this in, please look into doing the tiny doc upgrade we discussed earlier.

[KjellKod]

I'll go ahead and merge this and hopefully the docs will be updated by you or me before we forget about it.

[KjellKod]

This will be reverted. Ref #496 (review)