Pull request to add the new ECDSAIdentity

What this PR does

See commit message This PR

🙅‍ Friendly checklist:

[ ] 0. Code comments are added (or updated) when/where needed and explain the WHY of the code.
[ ] 1. Design choices, user documentation and any additional doc are added (or updated) in READMEs.
[ ] 2. Any new behaviour is tested and small units of code that can be are unit tested.
[ ] 3. Code comments are added on tests to explain what they do.
[ ] 4. Errors are systematically wrapped with a meaningful message using xerrors.Errorf and the %v verb.
[ ] 5. Hard limit of 80 chars is always respected.
[ ] 6. Changes are backward compatible.
[ ] 7. Indentation level does not exceed 5, although 4 is already suspicious.
[ ] 8. Functions, files, and packages are kept to a manageable size and decomposed into smaller units if needed.
[ ] 9. There are no magic values.

dedis / cothority