Revise cavity charged particle fluence calculation

Improved implementation of charged particle fluence calculation using
the proper expression for the path length due to a given energy loss.
One can choose between a 3rd or 5th order approach, as well as the
approach implemented in FLURZnrc.

Changes/additions:

- Approach A (stpwr) based on a general expression for the path length
  traveled when slowing down from initial energy Eb to final energy Ee as
  a function of the stopping power Lmid at Emid=0.5*(Eb+Ee)

- Add method getStepPerFraction to compute path per energy bin-width
  traveled by a charged particle when slowing down from initial energy Eb
  to final energy Ee

- Add option to select between O(eps^3) or O(eps^5) accuracy of approach
  A (stpwr) by using the input keys method = stpwr or method = stpwr05.
  Note that since both produce the same results in the examples tested,
  the O(eps^3) is set as default for being both, accurate and only 8%
  slower than the simpler FLURZ-mode approach B

- Pre-calculate values for faster evaluation of 1/Lmid[i] for the
  O(eps^3) mode and DE[i] for the log energy scale total fluence scoring.
  O(eps^5) would require more pre-computed values than just 1/Lmid. One
  requires for instance lnEmid[i] to get the b parameter and
  eps[i]=1-E[i]/E[i+1]. Not impossible, but seems unnecessary
  considering the excellent agreement with O(eps^3), which should be
  always used.

- Made algorithm work for logarithmic energy scale where the energy bin
  width is not constant which must be accounted for when determining the bin
  fractions and scoring the total fluence

- Scores total fluence accounting for correlations in the uncertainty
  while it also integrates the differential fluence at the end assuming bin
  contributions to be independent of each other.

- Clearer description of the fluence scoring method during output