Unique physics labels

[tommbendall]

This fixes our physics capability, to give us real control over which physics schemes are called and when.

This is done by:

• the Physics class is renamed as PhysicsParametrisation which will help us avoid confusion
• introducing a PhysicsLabel which is a type of new type of label, which actually gives a term two labels:
  • physics: with the value as a string describing the physics parametrisation
  • the unique label for that parametrisation, which stores the evaluate method as before
• ensuring that each PhysicsParametrisation object has a label
• when the TimeDiscretisation sets up its evaluate methods, it now ensures that they are unique
• a rogue extra call to evaluate physics schemes has been removed