Deepecho

This release fixes a bug with how DeepEcho handles NaN values.

* Handling NaN's bug - Issue [35](https://github.com/sdv-dev/DeepEcho/issues/35) by fealho

Maintenance release to update dependencies and ensure compatibility with the rest
of the SDV ecosystem libraries.

Minor maintenance version to update dependencies and documentation, and
also make the demo data loading function parse dates properly.

This version includes several minor improvements to the PAR model and the
way the sequences are generated:

* Sequences can now be generated without dropping the sequence index.
* The PAR model learns the min and max length of the sequence from the input data.
* NaN values are properly supported for both categorical and numerical columns.
* NaN values are generated for numerical columns only if there were NaNs in the input data.
* Constant columns can now be modeled.

Add BasicGAN Model and additional benchmarking results.

This release includes a few new features to make DeepEcho work on more types of datasets
as well as to making it easier to add new datasets to the benchmarking framework.

* Add `segment_size` and `sequence_index` arguments to `fit` method.
* Add `sequence_length` as an optional argument to `sample` and `sample_sequence` methods.
* Update the Dataset storage format to add `sequence_index` and versioning.
* Separate the sequence assembling process in its own `deepecho.sequences` module.
* Add function `make_dataset` to create a dataset from a dataframe and just a few column names.
* Add notebook tutorial to show how to create a datasets and use them.