Ensure const-correctness for graph element properties storage

[SpectraL519]

• Aligned the vertex/edge descriptor types to allow:
  • Proper implicit conversion from non-const to const variant
  • Comparison between a const and non-const variants
• Refactored the graph class and the internal implementation types to define and utilize const and non-const element descriptors based on the graph's constness
• Relaxed the c_graph concept to allow multiple value/type categories of the graph class
• Introduced helper typedefs to easily extract the typedefs from a graph type in generic contexts
• Refactored the internal specialized implementation types to use the explicit object parameter feature in order to be used as mixin base types instead of simple static method dispatchers
• Aligned the algorithm implementations to work with the new, aligned graph implementation

[SpectraL519]

ok

[SpectraL519]

ok