bugfix/cron-job-restarting-issue

[ndortega]

1. stopcronjobs() just stops all running jobs (doesn't clear jobs). This enables us to restart these cron jobs
2. added a new clearcronjobs() function which will wipe out any cron job definitions (will not impact running jobs)
3. It's no longer possible to add duplicate cron jobs (same name, function, and expression)

[codecov[bot]]

Codecov Report

Attention: 4 lines in your changes are missing coverage. Please review.

Comparison is base (299a561) 97.89% compared to head (a8cbc64) 97.69%.

Files	Patch %	Lines
src/cron.jl	87.50%	4 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #137 +/- ## ========================================== - Coverage 97.89% 97.69% -0.21% ========================================== Files 12 12 Lines 857 867 +10 ========================================== + Hits 839 847 +8 - Misses 18 20 +2 ```

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