Warning: assuming dim tags are same with different size placeholders - make exception with new behavior version

[albertz]

          # Note: Instead of making this a warning, we could also enforce this at some point.
          #   The user should be able to fix `extern_data` in the config such that this is correct in the first place.
          #   Also, in addition to this warning, we might want to add some runtime check on the eq of the dyn sizes.
          print(
            "Warning: assuming dim tags are same with different size placeholders: %r vs %r" % (
              self.dyn_size, other_same_base.dyn_size))

This should be an exception instead, not just a warning. For newer setups, this should never be allowed.

This requires a new behavior version (#508).

[Dark-Rock]

Hey can I work on this issue?