Hpl-specs

Changed
- Python 2 is no longer supported.
- Modernized tooling and package structure.
- Improved API in various aspects, with some backward-incompatible changes.
- AST objects are mostly immutable, with available methods to create mutated copies.
- Added a number of attribute validators for AST data types.

Fixed
- Fixed a bug with properties that had complex events such as disjunctions.

Fixed
- `MANIFEST.in` was missing `requirements.txt`, causing an issue when installing with `pip`.

Added
- More builtin functions to use with `HplFunctionCall`.

Changed
- `HplEventDisjunction` requires unique ROS names for each disjoint event.

Fixed
- `HplProperty` sanity checks for Requirement pattern.

Added
- Convenience methods in `HplPredicate` to check references to aliases.
- Event disjunctions; e.g., `(a or b) causes c`.
- Convenience methods to duplicate `HplAstObject` instances.
- `logic` module with some convenience, logic-related functions.