math: improve diag_value constructors

[andrei8l]

Summary

None

Purpose of change

diag_value and diag_kwargs are a little awkward to use

Describe the solution

Add more constructors so you can use arithmetic values and string literals directly

Instead of

    diag_value type( std::string{ "ALL" } );

you can now write

    diag_value type( "ALL" );

Instead of

    diag_value type( static_cast<double>( PICKUP_RANGE ) );

you can now write

    diag_value type( PICKUP_RANGE );

Instead of

    diag_value str_val( std::string{} );
    diag_value art_val( 0.0 );

you can now write

    diag_value str_val;
    diag_value art_val;

and they are treated as zero/null values as needed.

You can also easily construct arrays now

    diag_value val( diag_array{ 1, 2, "three", var_info{} } );

Add a `kwarg_or()` helper function

Instead of

    diag_value type( std::string{ "ALL" } );
    if( kwargs.count( "type" ) != 0 ) {
        type = *kwargs.at( "type" );
    }

you can now write

    diag_value type = kwargs.kwarg_or( "type", "ALL" );

Works with empty values too

    diag_value type = kwargs.kwarg_or( "type" );

Describe alternatives you've considered

N/A

Testing

Existing test units

Additional context

GuardianDll take a look please and let me know if this is clear.

[GuardianDll]

Yeah looks great for me