setData + getData functions

[YamenMerhi]

Description

Changed the setData and the getData functions in the ERC725Y contract.

@frozeman , @CJ42 and I found that the better way to setData is by passing an array of data which can be set in one transaction, that way we can skip the need to to make a contract call each time we want to set singular form of data. Same applies for getData.

Changes

• Added Arrays parameters to the setData , getData functions in ERC725YCore.sol , ERC725Y.sol , ERC725YInit.sol and IERC725Y.sol
• Edited the docs.
• Changed the ERC165 interface Id.
• Added a require statement in setData function
• Optimized getData function
• Added 2 dummy contracts used for tests to ./helpers
• Added Tests covering changed functions.