Qdscreen

- Replaced usage of deprecated `scipy_mode`. Fixed [39](https://github.com/python-qds/qdscreen/issues/39)
- Fixed `ValueError: invalid literal for int() with base 10` in `predict_qd`. Fixed [40](https://github.com/python-qds/qdscreen/issues/40)
- Added input validators to raise human-readable error messages when the input is not correct. Fixes [37](https://github.com/python-qds/qdscreen/issues/37)
- Fixed `AttributeError: module 'numpy' has no attribute 'object'.`. Fixes [38](https://github.com/python-qds/qdscreen/issues/38)

- Fixed `ValueError` with recent versions of `SciPy`, due to usage of sparse arrays with object dtype. Fixes [31](https://github.com/python-qds/qdscreen/issues/31)
- Fixed `IndexError` when `NaN` values are present in the dataframe. Fixes [28](https://github.com/python-qds/qdscreen/issues/28)

- Now filtering `UserWarning` in `fit_selector_model` even in the sklearn adapter. Fixes [20](https://github.com/python-qds/qdscreen/issues/20)

- Now filtering `UserWarning` in `fit_selector_model`. See [20](https://github.com/python-qds/qdscreen/issues/20)
- New build system: now using `virtualenv` instead of `conda` in `nox` sessions. Fixes [23](https://github.com/python-qds/qdscreen/issues/23)
- New project layout to avoid bug with `xunitparser`. Fixes [18](https://github.com/python-qds/qdscreen/issues/18)

- Migrated CI/CD from Travis to Github Actions + `nox`.

- `selector_skl` module renamed `sklearn` and `QDSSelector` renamed `QDScreen`. Fixes [16](https://github.com/python-qds/qdscreen/issues/16)

Initial release with:

* A main method `qd_screen` to get the (adjacency matrix of) the quasi-deterministic-forest, a `QDForest` object with string representation of arcs (Fixes [8](https://github.com/python-qds/qdscreen/issues/8)).
* Possibility to `keep_stats` so as to analyse the (conditional) entropies in order to define a "good" threshold.
* A method `<QDForest>.fit_selector_model(X)` to fit a `QDSelectorModel` feature selection model able to select relevant features and to predict missing ones. Fixes [7](https://github.com/python-qds/qdscreen/issues/7)
* Support for both pandas dataframes and numpy arrays as input. Fixes [2](https://github.com/python-qds/qdscreen/issues/2)
* A Scikit-learn compliant feature selector `QDSSelector`, providing the exact same functionality as above but compliant with scikit-learn `Pipeline`s. Fixes [1](https://github.com/python-qds/qdscreen/issues/1)

Non-functional:

* Travis continuous integration, generating documentation and deploying releases on PyPi
* A package level `__version__` attribute. Fixes [3](https://github.com/python-qds/qdscreen/issues/3)
* Added `py.typed` for PEP561 compliance. Fixed [4](https://github.com/python-qds/qdscreen/issues/4)
* Initial `setup.py` and `setup.cfg`