Fix beamdp axis labels

[blakewalters]

Addresses #1004.

Note: I find our method for specifying axis units in beamdp using a backslash in front of the unit (i.e., '/unit') confusing, especially for quantities which may also have a unit in the denominator (e.g., '/MeV cm^-2'). I wonder if we should move to just using brackets instead.