feature/repeat-tasks-and-cron-internals-refactor

[ndortega]

• Added a @repeat macro to register repeat tasks
• Now utilizing an internal Nullable type to simplify type definitions
• The pathing used in Instances.jl now supports windows
• Added concrete types to the repeat tasks & cron state variables
• Added a new section in the docs for multiple instances

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 97.49%. Comparing base (c61f3ad) to head (7cbf784).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #166 +/- ## ========================================== + Coverage 97.34% 97.49% +0.15% ========================================== Files 20 20 Lines 1129 1157 +28 ========================================== + Hits 1099 1128 +29 + Misses 30 29 -1 ```

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