GitLab

- Upgraded parent project to v7.6.1
- Changed to CloseablePolicyProvider.Factory#getInstance(...): new
parameter 'otherHelpingPolicyProvider': adds support for multiple Policy
Providers combination, i.e. a Policy
Provider may call another policy provider - during instantiation or
later - to resolve policies (references) it cannot resolve on its own

an actual implementation should be used instead at runtime, e.g.
com.sun.mail:javax.mail

- updated license headers for year 2020
- Changed PolicyProvider interface : new method getCandidateRootPolicy()
to provide a default root policy when 'rootPolicyRef' undefined in PDP
configuration