Add automatic display in notebooks when MolSysView is the last expression

[dprada]

We want MolSysView to display automatically in Jupyter when the object is the last expression in a cell, without requiring an explicit .show() call. This is to keep molsysmt.basic.view() returning the MolSysView object (no forced show) while still giving a good notebook UX.

Proposed options:

1. Implement _repr_mimebundle_ (or _repr_html_) in MolSysView to delegate to self.show() or return the widget’s mimebundle. This would make MolSysView render automatically when evaluated in a notebook cell.
2. Alternative: expose a .widget attribute and implement __repr__/_ipython_display_ to forward to the widget’s display.

Constraints:

• Must not trigger display on object creation (only when evaluated as the last expression).
• Keep .show() behavior unchanged (first call returns widget, subsequent calls return None).
• Do not add extra dependencies.

Expected outcome:
MolSysView renders automatically in Jupyter when it’s the last expression, while still being a plain Python object in scripts.

[dprada]

This was already implemented.