Separate initialization call from the constructor

[rya-sge]

See #59

Some references for a better understanding :

• What’s the difference between onlyInitializing and initialzer?
• ERC20MockUpgradeable
• Update initializer modifier to prevent reentrancy during initialization

[rya-sge]

we have to use onlyInitializing in the internal functions and the modifier Initializer in the public function for all contracts.