Scalar return values are now invalid

[philip-paul-mueller]

Due to how the code generator works a scalar return value is not possible and it does not generate an error.
Returning a scalar is a desired feature as there is code in CompiledSDFG for that, but it does not work.
The frontend works around this by simply promoting everything to an one dimensional array.

This commit introduces:

• A change to validate to reject such SDFGs.
• A test that can test the feature, but is disabled, since it does not work.
• A test for the new restriction.