Bring the solidity code up to date with the coding standards

[AntoineRondelet]

The PR #318 added some basic and missing code standards for the solidity code. Several files are not compliant with it (in fact, as of now some contracts code styles conflict) and follow different conventions. Let's bring all these files up to date with the solidity code standards to have a consistent code.

In the same time, we may want to use solhint (https://github.com/protofire/solhint) as solidity linter to enforce best practices (code/style) and check these on the CI server.

[AntoineRondelet]

Despite #333 being merged, this ticket remains opened since the smart contract code is not fully compliant with the solidity standards (doing so would have cause too many changes for a single PR). This will be done incrementally. As such, this ticket remains until the whole code satisfies the solidity coding standards.

[AntoineRondelet]

#361 has brought the contract code up to date with the solidity standards overall. We made amendments to the coding standards of solidity to fit our needs, but the functions' interfaces etc are now compliant.
Closing this issue as a consequence.