follow-up tasks for the multi-dimensional plotting feature for `explore`

[keewis]

In #124, we've added sliders to support plotting arrays with more than the spatial dimension. This is not yet complete, though we still have a few things to do:

• figure out how to update the pixel inspection table with the currently displayed values
• select using coordinate values, not just integer indices
• tests (plotting tests #127)
• decide how to format the control panel (and where to put it)
• add an example to lonboard's integration gallery

[tinaok]

Can we have quick example of multi-dimentsional plotting here?