Check coords are identical in slicing dimension in instrument view

[nvaytet]

If you passed dg.hist(tof=100) to the instrument_view, the different data arrays would have different tof coords, and the view would fail with a very strange error:

File ~/development/venv4essdiffraction/.venv_scipp_may26a/lib/python3.14/site-packages/plopp/widgets/slicing.py:155, in <genexpr>(.0)
    153 new = [min(max(x, self.min), self.max) for x in change["new"]]
    154 self._lock = True
--> 155 self._widget.value = " : ".join(f"{self._coord[x]:{self._fmt}}" for x in new)
    156 self._lock = False

TypeError: unsupported format string passed to numpy.ndarray.__format__

We add a safety net to catch this before we attempt to make the plot.

@celinedurniak