Closes #44

Co-authored-by: Confiks <confiks@scriptbase.org>

Co-authored-by: Confiks <confiks@scriptbase.org>

Co-authored-by: Confiks <confiks@scriptbase.org>

Attributes may now also remotely be made optional.

When a problem occurs during parsing or verifying a scheme manager or any of its underlying files, this is now stored in the scheme manager struct itself instead. Invalid managers are kept in Configuration.SchemeManagers.

Authenticity of the scheme manager files in the irma_configuration
folder is now checked as follows.
Each scheme manager must henceforth include three new files in its
root folder: index, index.sig, and pk.pem. index lists all files
under this manager as well as their SHA256-hash. index.sig is a
ECDSA signature over index, signed with the private key corresponding
to the public key pk.pem. Thus, a file under the scheme manager
is authenticated if its hash matches the one in the index file,
and if the signature over the index file is valid.

Authenticity of all xml files, including the Idemix public keys,
is verified in this way automatically when these files are loaded.
For issuer and credential logos, use Configuration.ReadAuthenticatedFile.