Leap-ec

* New features
* Added support for static- and variable-length segments, which are fixed-length "chunks" of values
* Added support for integer representations, via the `int_rep` package
* Added a simple neural network representation, `executable_rep.neural_network`, and made it the default for `examples/openai_gym.py`
* Added a Cartesian genetic programming (CGP) representation, `executable_rep.cgp`, with example in `examples/cgp.py`
* Added support for heterogeneous island models, demoed in `examples/multitask_island_model.py`

* CI/CD
* Greatly improved test coverage
* Added `statistical_helpers` to assist with writing unit tests for stochastic algorithms

* API changes
* Added probability parameter for the `n_ary_crossover` operator
* Changed the `Executable` interface to act as a `Callable` object (rather than using a custom `output()` method)

* Significantly added to online [documentation](https://leap-gmu.readthedocs.io/en/latest/index.html)
* Major code reorganization
* exception management for `Individual` has been moved to `RobustIndividual`
* `DistributedIndividual` now inherits from `RobustIndividual`
* `core.py` has been broken out to separate modules
* `Individual` and `RobustIndividual` now in `individual.py`
* representation specific entities moved to new sub-packages, `binary_rep`
and `real_rep`
* `Representation` now in `representation.py`
* `Decoder` now in `decoder.py`
* documentation, doctests, examples, Jupyter notebooks, and unit tests updated accordingly
* added ability to pass ancillary information during evaluation, such as UUIDs
that could be used to name output files and directories, yet do not have a
direct impact on fitness

* Apply `Representation` consistently throughout LEAP, particularly the top-level monolithic functions
* Added probe to `leap_ec.distributed.asynchronous.steady_state()` to take regular snapshots of the population

* fix how non-viable individuals sort themselves when compared since the prior method of comparing `math.nan` to `math.nan` yielded non-ideal behavior
* minor maintenance tweaks

* changed package name to `leap_ec` from `leap` to mitigate pypi namespace collisions
* minor maintenance tweaks

* first major "mature" release of LEAP