use available resources in examples

[GregorDeCillia]

• [ ] Make sure all codes used in the documentation (examples and articles) can be run with any API key.
• [ ] Also check, whether the json requests in inst/json_examples are restricted to certain user groups and replace them accordingly.
• [x] The function sc_example() should be exended so a list of all available examples can be displayed. Either do this with a function like sc_examples_list() or display an error message with availale examples if sc_example() is called with an invalid argument.

[GregorDeCillia]

I think it is pretty much impossible to tell who has access to which cubes and which variables. Therefore I'll mark this as closed. However, free to comment on this issue if there are any test requests in sc_example() or in the docs that cannot be used with a valid API key.