Tarski

Changed
- Almost-identical methods `approximate_symbol_fluency` and `classify_symbols` have been merged into one
single method `approximate_symbol_fluency`.

Added
- Preliminary [readthedocs documentation](https://tarski.readthedocs.io) integrated in the repository.
CI tests the documentation build as well.
- Integration with the [PySDD package](https://github.com/wannesm/PySDD) for sentential decision diagrams
to process action schema preconditions.
- Implementation of a `project_away_effect_free_variables_from_problem` transformation that for each action schema
compiles into existential variables all action parameters that are not used in the action effects
([63](https://github.com/aig-upf/tarski/issues/63)).
- Implementation of a `compile_universal_effects_away` transformation that expands universal effects in actions.
- Reachability module now processes problems with cost-related functions (010d79df)
- Preliminary implementation of a library of benchmark generators
([43](https://github.com/aig-upf/tarski/issues/43)).
- Added some preliminary support for the NDL representation language.

Fixed
- Fixed some minor bugs in FSTRIPS writer.
- Fixed bug in ReachabilityLPCompiler when problem has an action and a predicate with the same name (7e9a684).
- Remove temporary files created by the LP based grounder.
- Model.list_all_extensions now returns empty extensions if necessary (ffbc96d1)

Added
- Preliminary implementation of a `tarski.fstrips.representation` module with some representational queries
and transformations.
- Preliminary integration with [mypy static typing analysis](https://github.com/python/mypy) which is
checked now in the CI tests.
- Add helper function `find_domain_filename` to infer domain filenames from instance filenames.
- Add `collect_unique_nodes` method to collect all AST nodes of any FSTRIPS expression.
- Add namespace accessor `lang.ns` to FOL objects to allow direct access to any language element.

Fixed
- Fixed bug with fluent / static symbol classification [66](https://github.com/aig-upf/tarski/issues/66).
- Fixed bug with multiple conditional effects in [FSTRIPS / PDDL parser](https://github.com/aig-upf/tarski/commit/c89ac31623171b78689d5d0ae3eca07c2be2ad71).
- Fix bug in printing of FSTRIPS instances [69](https://github.com/aig-upf/tarski/issues/69).
- Some other minor bugfixes.

Added
- Parsing and writing of Functional STRIPS and PDDL encodings of classical planning problems.
- Parsing and writing of RDDL encodings of Markov Decision Processes (MDPs) and hybrid planning problems.
- Answer Set Programming-based grounding and reachability analysis for classical planning problems.
- Description Logics module for classical planning problems.
- Support for Existentially and Universally Quantified effects in Functional STRIPS and PDDL domain descriptions.
- Syntax for elementary linear algebra operations with vectors and matrices.
- Support for the evaluation of expressions involving matrices, vectors and scalars.
- Expression simplification.

First public release.