Tidied up integrate.cpp a bit
More tidying of integrate.cpp
Added GenPopulations and GenPSD functions

GenPopulations estimates the number of particles which should belong
in each size class according to the method described in ch 5.4 of
[RAD11].

GenPSD takes the populations and using eqn. 5.7 from ch. 5.4 [RAD11]
generates a distribution of particle sizes using a Mersenne twister to
produce pseudorandom numbers. These sizes are then sorted using a
function called SelectionSort (www.codebeach.com/2008/09/sorting-
algorithms-in-c.html).