Adding a 'makeyourown' kernel tutorial, and a 'varying biofouling parameters' tutorial.

[michaeldenes]

Pull request to add two new tutorials to the examples folder.

[codecov-commenter]

Codecov Report

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

Project coverage is 15.43%. Comparing base (78ba7e5) to head (033b97a).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #32 +/- ## ======================================= Coverage 15.43% 15.43% ======================================= Files 6 6 Lines 473 473 ======================================= Hits 73 73 Misses 400 400 ``` | [Flag](https://app.codecov.io/gh/OceanParcels/plasticparcels/pull/32/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=OceanParcels) | Coverage Δ | | |---|---|---| | [unit-tests](https://app.codecov.io/gh/OceanParcels/plasticparcels/pull/32/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=OceanParcels) | `15.43% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=OceanParcels#carryforward-flags-in-the-pull-request-comment) to find out more.

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