Layered Generator

[astrsh]

Pull Request

Moved from #344

Brief description.

What Issues Does This Fix?

Implements a new generator based on 'layers' as outlined in #340

Licensing

• I am the original author of this code, and I am willing to release it
  under GPLv3.
• I am not the original author of this code, but it is in public domain or
  released under GPLv3 or a
  compatible license.

Goal of the PR

See #340

Affects of the PR

Types of changes

• Bug Fix
• Build system
• Documentation
• New Feature
• Performance
• Refactoring
• Repository
• Revert
• Style
• Tests
• Translation

Compatiblity

• Breaking
  change
• Non-Breaking change.

Contribution Guidelines.

• I have read
  the CONTRIBUTING.md
  document in the root of the git repository.
• My code follows the code style for this
  project.

Documentation

• My change requires a change to the documentation.
• I have updated the documentation accordingly.

Testing

• I have added tests to cover my changes.
• All new and existing tests passed.