Hack to make MultipleScatteringParametrisation thread safe

Converted the static being modified to be thread_local. However, this
is really just a hack since the static is being used as a 'global'
cache of EventSetup data which is not allowed. This change allows
us to move forward until an actual acceptable change can be made.
Merge pull request #3512 from Dr15Jones/hackThreadSafeMultipleScatteringParametrisation

Multithreading -- Hack to make MultipleScatteringParametrisation thread safe