Enable the possibility not to define a TimeManager in a Model
A Model
relies on a base structure: for instance it heavily assumes a TimeManager
is defined.
This is for very good reasons for a a full fledged Model, but is rather cumbersome for tests as we need to clutter the Lua files with unused data (time manager to begin with for most of them).
A small behind the hood refactoring should be done so that full fledged Model behaves exactly the same but test ones may use a slighter interface.