Threads for wrapper are now disabled by default

See merge request !417

Disabled threads for the wrappers, added an option to enable them. Refactored the function, added docs

Now, wrappers admits %Placeholders%

Fixes check: "on_submission" for wrappers and total_jobs when set at job_level

Closes #1255

See merge request !414

Fix "previous" when there is only a single job

See merge request !415

Calendar splits - rebased

Closes #1158

See merge request !407

Get rid of DeprecationWarning: pkg_resources is deprecated as an API

See merge request !416