Add feature where an externally generated attenuation profile can be loaded into
the material definition from a .DAT text file. This will override the normal
exp(-alpha*thickness) calculation of the attenuation. The new feature was requested
as part of the absorption calculation for the Pearl instrument where some parts
of the environment are crystalline

Found the lookup\interpolation on the loaded profile was slowing down the absorption
Monte Carlo calculation.
So improved performance of Kernel::Interpolation. Changed it to use vector of pairs
instead of two separate vectors to reduce amount of navigation around vectors

Considered adding feature into AttenuationProfile where I add any interpolated points
into the override attenuation profile to prevent interpolation next iteration of
Monte Carlo simulation. However wasn't convinced this was going to be thread-safe
during the parallel part of the absorption calculation - may revisit this. While doing
this investigation I fixed the following issues with the function Interpolation::AddPoint:

a) it doesn't add the same point twice
b) it reuses the lower_bound function rather than stepping through the vector sequentially

I removed the exception from findIndexOfNextLargerValue because the end()
iterator is a valid result if the x value supplied equals the last x value
already in the vector. Could have left check in for begin() but didn't seem
much point

Finally - the interpolation outside the range is not v good. The data at the extremes
is noisy so we end up extrapolating based on random gradient in extreme 2 points. Need
to either fit or use more points to determine gradient