Singletons are classes which are instantiated once and only once.

Most object-oriented books are not in favour of singletons, as they disagree with that idea that there's a class definition with multiple instantiations of this class which are called objects. However, Peano needs singletons in several cases.

Singleton naming conventions

Singletons are realised by making all constructors private or protected.
A singleton has a dedicated function
```
    static MyType& getInstance();
```
which returns a non-const reference.
Its implementation holds a static reference to a singleton object which is then returned.

MPI services

Due to Peano's architecture, there are classes which need to poll MPI every now and then for potential messages (such as load balancing notifications). Such classes are called services. Services

are always singletons;
are always inheriting from tarch::services::Service.