update plot-energy to plot energy bias and resolution for stopping muons

To select stopping muons we simply look for the muons before a Michel event.
The muon distance is calculated by first projecting the muon fit back to the
PSUP along the fitted direction and then taking the distance between this point
and the fitted position of the Michel event. I then calculate the expected
kinetic energy of the muon by using the muon lookup tables of the CSDA range to
convert the distance to an energy.

I also changed a few other things like changing as_index=False ->
group_keys=False when grouping events. The reason for this is just that if we
do this we don't have to reset the index and drop the new index after calling
apply().

I also fixed a small bug I had introduced recently where I selected only prompt
events before finding the michel events and atmospheric events.

This commit updates the plot-energy script to plot the energy bias and resolution for stopping muons by computing the expected

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