Stimuli

- Cast loaded sounds to `np.float64` to prevent floating point error in the volume assessment.

- Improve packaging
- Fix indentation in docstrings with a summary line
- Add dependabot to update github actions
- Fix casting of true_divide with different dtypes when loading a sound

- Better handling of blocking audio stimulus
- Add DOI

- Add a CLI entry-point to display a fixation cross: `stimuli_cross`. The cross settings can be changed with the command-line arguments. Use the flag `-h` or `--help` for additional information.
- Rename ASSR sound stimulus to SoundAM, as this sound stimulus is not restricted to an Auditory Steady-State Response.
- Replace WhiteNoise with Noise which supports colored noise.

Initial release with auditory stimulus class for:
- Pure tone stimulation.
- White noise stimulation.
- ASSR stimulation.
- .wav sound files.

and with visual stimulus class for:
- Text
- Fixation cross
- Horizontal or vertical moving bar
- Horizontal or vertical filling bar, i.e. double-side thermometers.