* Add another method of tracing Simulink models via [block
properties](https://www.mathworks.com/help/simulink/ug/block-properties-dialog-box.html).
* The top-level system of a Simulink Library (but not Model) no longer
appears in the tracing output, as there is no way to access or
manipulate this in the Simulink user interface.
* Add new option `--untagged-blocks-inherit-tags` for `mh_trace` which
cases any untagged system to inherit any tags from its parent. Once
new tags are encountered this breaks the inheritence chain. Note
that embedded MATLAB blocks are unaffected by this option.