Monte Carlo simulation to calculate a quantity called absorption
weighted path length that is used in extinction correction calculations
downstream from Mantid eg in Jana. Requested by WISH. Specific changes:

- new AddAbsorptionWeightedPathLengths algorithm that uses the existing MCAbsorptionStrategy class to do most of leg work
- adjust the MCAbsorptionStrategy class to make the code more reusable for calculation involving a single wavelength such as the absorption weighted path length for a peak. In particular change the Calculate function signature to replace the HistoramData and Spectrum parameters with simple vectors and move the interpolation functionality back into the MonteCarloAbsorption algorithm
- extend the Peak class to add in an extra column called tbar for the absorption weighted path length + get\set functions. Also expose the get\set in the Python interface
- update SaveReflections python algorithm to write the actual tbar value for each peak in a peaksworkspace out to Jana file instead of zero
- add the absorption weighted path length to the nexus file and adjust save\load code