one to many relationship unit tests

[Felecarpp]

Hello,
I need to create relationships between my entities in my project. I have read this thread (#62) and I would like to propose a way to build them using esper.
The key idea is to add a same component instance (called a group) to many entities. This works already fine with esper but not any test assure it over the long-term. So this PR only adds unit tests.

[Felecarpp]

Finally, I found another way to implement it which don't need to add the same instance of a component to many entities.

the first idea

item 1		character		item 2
Inventory	<--	Hold		Inventory (the same instance)

the new one

item 1		character		item 2
Belong	-->	Inventory	<--	Belong

for the farm/plant question (#79)

plant 1		farm		plant 2
Rooted	-->	Plantation	<--	Rooted

The only oddity remaining is that a component has another component as an attribute. This allows to do not use an entity ID instead or to use a complex identifier system.