[Task] running individual pcse module with `TestEngine`

[SarahAlidoost]

Currently, the only way to run an individual pcse module e.g. leaf_dynamics is to use TestEngine that is not a part of pcse itself, but stored in the folder tests in pcse repo. The TestEngine supports two main functionalities that are currently missing in the Engine class of pcse:

• accepting external dependencies through kiosk variable [To be done]
• accepting model config as an object rather than a path to a config file ✔️

For more info, see the tests implementation in #12. When working on #19, consider this issue.

[SarahAlidoost]

We want to add support for first item. The second point have been already implemented.