The unit test suite MonteCarloAbsorptionTest has occasionally been failing
with an access violation on Jenkins. I've been able to track this down to
the test Lambda_StepSize_Two_Linear_Interpolation which was failing

I can reproduce this locally by modifying the test to repeat 1000 times
and it eventually fails. Although the access violation occurs in a couple
of different places:
a) the y() or e() vector in the 0th histogram is sometimes empty once the
MonteCarloAbsorption algorithm completes which causes some of the test asserts
to fall over because they assume y(0).front() or e(0).front
b) sometimes the code in MonteCarloAbsorption itself has an access violation
when yIndexOfX returns an index out of range of the histogram

Both problems happen in the multi-threaded code in MonteCarloAbsorption and
it seems to be the interaction between the call to MatrixWorkspace::yIndexOfX
and the code that updates a histogram with interpolated results via
MatrixWorkspace::setHistogram. Something in these two functions isn't threadsafe
I found that yIndexOfX calls IsHistogramData() which checks the 0th
histogram in the workspcae even if the MonteCarloAbsorption code is
processing the 1st, 2nd etc workspace
I think if the 0th histogram is in the process of being updated by another
thread then yIndexOfX can fail or else the setHistogram call fails.
I have done two changes:
- only call yIndexOfX once (there was a redundant 2nd call)
- I have modified yIndexOfX so it takes an additional histogram index parameter.
This allows me to keep each MonteCarloAbsorption thread interacting with a
single histogram only. I have defaulted the new parameter to zero so other
code behaves as before