add licence headers

[alexcos20]

Closes #457

Changes proposed in this PR:

• add licence headers

@trentmc @kremalicious @lacoop6tu - need you double check pls

PS: Artifacts & deployment are not included, will do that after several PRs

[kremalicious]

seems alright, only the parentheses for the SPDX-License-Identifier might not be needed when looking at https://spdx.dev/ids/#how as they seem to only use this for more than 2 licenses. As this field is supposed to be machine-readable would be cool to have some tool to check if licenses can be read out correctly

[alexcos20]

solidity compiler already does this, we have no more warnings