sdk: Add tutorial for Submodel navigation

[moritzsommer]

Previously, we lacked a tutorial demonstrating how to navigate a Submodel's hierarchy using IdShorts and IdShortPaths.

This adds a simple tutorial for Submodel navigation and updates the documentation to reference it. The end user is shown how to navigate SubmodelElements, such as simple Properties, Property Collections, Property Lists and Collection Lists.

Fixes #351

[moritzsommer]

I have two more questions about this topic:

1. When using get_referable(), the user gets back a Referable object. This object does not have a value attribute, so in order to access this attribute (in a clean way, without mypy complaining), you have to cast. Is this behaviour intended?
2. In the last dev meeting, we didn't cover the topic getting objects back out of an ObjectStore. Therefore, I didn't include it in the tutorial. However, Add tutorial aas_navigation  #351 and Usage of id_short Path Parameter in get_referable #345 mention it. Should I include the workflow presented in Usage of id_short Path Parameter in get_referable #345 in this tutorial?

[s-heppner]

@moritzsommer wrote here:

1. When using get_referable(), the user gets back a Referable object. This object does not have a value attribute, so in order to access this attribute (in a clean way, without mypy complaining), you have to cast. Is this behaviour intended?
2. In the last dev meeting, we didn't cover the topic getting objects back out of an ObjectStore. Therefore, I didn't include it in the tutorial. However, Add tutorial aas_navigation  #351 and Usage of id_short Path Parameter in get_referable #345 mention it. Should I include the workflow presented in Usage of id_short Path Parameter in get_referable #345 in this tutorial?

ad 1.) I agree that this is ugly, but not every Referable has a value in the meta-model and we need to follow the specification also for the abstract classes. Do you have a better solution than what we have now?

ad 2.) This should be covered by the tutorial_storage. Maybe you can refer in this tutorial to it?

[moritzsommer]

ad 1.) I agree that this is ugly, but not every Referable has a value in the meta-model and we need to follow the specification also for the abstract classes. Do you have a better solution than what we have now?

ad 2.) This should be covered by the tutorial_storage. Maybe you can refer in this tutorial to it?

1. Unfortunately, I think casting is the only proper option here. Any other approach would lead to Mypy complaining.
2. All right, I haven't changed anything about the content of the current tutorial.

[s-heppner]

LGTM, thank you very much!