Harmonize execution option with ewoksdask

[poautran]

In GitLab by @woutdenolf on May 25, 2023, 13:05 GMT+2:

pool_type -> scheduler

scaling_workers -> scheduler_options["scaling_workers"]

max_workers -> scheduler_options["max_workers"]

context -> scheduler_options["context"]

Also document like for ewoksdask: https://ewoksdask.readthedocs.io/en/latest/parallel.html

Migrated from GitLab: https://gitlab.esrf.fr/workflow/ewoks/ewoksppf/-/issues/14