_verifier: address of the verifier for the new verification method
_masterAddressis verified by
_masterAddressmustn't already be registered with
_masterAddress: Address of the user being verified
_masterAddresswill be usable to perform other functions (such as identifying users registered with a verifier). If
_masterAddresswill purely be considered for registering the user. Further addresses associated with the
linkAddress()will only be considered for any activities pertaining to verification.