Rewrite STDCM Allowances component

[nicolaswurtz]

STDCM Allowances were based on old allowances component from operational studies.
STDCM just needs 3 input fields to enter allowances settings, so it would be straightforward to implement a lightweight solution rather than use a huge component.

New component must give the same functionalities as today but be entierely new.
It has to propose 3 input fields :

• margin before
• margin after
• global allowance (% or min/100)

It should use the already existing components from OSRD InputSNCF for the two first fields, and InputGroupSNCF for the last field. Implementation from new allowance component from operational studies could be a good reference, but no dependence will be tolerated.

OldAllowances folder will be fully destroyed and thrown into the abyss of the void at the end of this refactoring process.