Write vignette demonstrating the effect of parallelisation

[gregorgorjanc]

• Show the effect of parallelisation on time
• Describe how to limit the number of cores used (setting SP$nThreads to less than max)
• Describe how AlphaSimR uses OpenMP to speed up operations on large populations (we still get this in SIMplyBee when working with single colony and its castes) and how SIMplyBee used foreach/doParallel to speed up operations on large number of colonies
• See also thoughts on using both OpenMP and foreach/doParallel together at Explore using foreach/doParallel and OpenMP jointly #615