Add some safety checks in Template.add_template_part

[SylvainTakerkart]

It would be good to add some safety checks in this method, by adding a test on the group_id of the template-part that is added: check that this group-id does not exists in the current template; what happens if group-id already exists? etc. etc.

[sifaoufatai]

it clearly done in the algorithme

[SylvainTakerkart]

I understand that at the moment, the only algorithm that uses this method ensures that there should be no problem... but think about the future when some other developer will add a new functionality and will want to use this method; that is where we need it!

Therefore, it is necessary to add such safety checks!