Constexpr additions

[JustCallMeRay]

I have added constexpr specifiers to Node and Edge classes. These allow for the specified functions and objects to be ran at compile time instead of runtime, resulting in a speed increase.

A possible use case that might result in a speed up:

// load a pylogenic tree from 3 databases
// Base nodes can be constructed at compile time
Node<Enum, Organism> bacteriaBase {  Enum.BacteriaBase, Organism{} };
Node<Enum, Organism> bacteriaBase {  Enum.Archea, Organism{} };
Node<Enum, Organism> bacteriaBase {  Enum.Eukaryota, Organism{} };
// Edges can also be constructed at compile time
Edge ...
// Code cannot be run at compile time so everything after this line cannot be sped up
std::vector<Node<Enum, Organism>> nodes = LoadFromDatabase();

I have changed the UserId to be able to take anything as a template specialisation which may have a possible pain point with const char* being picked over strings and therefor doing a shallow copy over a deep string copy, so I have a static assert to make sure that never happens.