Rename `PublicKeyPair` to `PublicKeyDouble`

[moCello]

This commit also

• restructures some of the code
• renames the method: sign_single to sign
• add more latex rendered documantation to the README

Resolves: #110

[moCello]

I decided to name the double public key PublicKeyDouble instead of DoublePublicKey.

This is in anticipation of a later issue, where we introduce a public key with variable generator with the name PublicKeyVarGen #107