check-block: ignore issuer

To verify if our node will success to forge an acceptable block, we can use:

duniter gen-next --show --check --submit-local

This command will display the block and check it, signature and pow excluded (to have a quick answer about the block's data, which is what we want to check).

However, the issuer still needs to be a member. So our node must a valid member key. This issues aims at changing this point.