Comment changes & minor performance tweak

Update the big docstring at the top of the file and also one minor
performance improvement: calculate num_events in one line instead of 2.
Updates to the basic_variables plugin

1) Use a much faster technique for calculating the accumulated proton
charge
2) Add a function for a new PV: CALCULATED_POWER.  This is the power
delivered to the target calculated from the proton charge (and a few
assumptions).  Mainly useful as a sanity check on the proton charge
value.