Update submitter for pre/post tasks

[Alxiice]

Description

This PR aims to update the submitter so that it works with the pre/post task feature in Meshroom.
It requires this PR in order to work.
Make sure to follow these steps :

• merge Temporary correction to make sure we don't break the submitting process #4
• merge Meshroom!2984
• merge the current PR

Content of the PR

Changes :

• All the task chain logic is moved in Meshroom.
• New structures are used to store a concept of "OrderedTask" in Meshroom.
• Meshroom handles the "cooking" part that was here previously : it will build a Graph-like object that holds all the tasks & dependencies. All the logic concerning expanding/chunk/preprocess/postprocess/placeholder tasks already exist in this new OrderedTasks object.
• Now the submitter "simply" has to take this object and straightforwardly create tasks one by one, then add dependencies, then submit. No complex cooking/reordering task is involved here anymore

Additions :

• The concept of "task type" has been fully implemented.
• We also added the handling of preprocess/postprocess tasks.
• Also there is the concept of "placeholder" tasks : tasks that don't have any command but are here to create nodes in the graph
• The tractorSubtaskWrapper script has been renamed to tractorExpander to make it more general-purpose.

Removal :

• For now we removed the chunk grouping : every chunk will have it's own task. Grouping chunks on multiple tasks have no real purpose here as we already have a size concept in the chunks. That was also a feature that we didn't use so there was no reason to keep it.