developments from TW - still testing!!

possibility of comparison with W&P (2009) added;
collision models added - simple model runs and gives right pattern but
needs a very low restitution coefficient.

Still to be tidied and checked more.

Looking into applying some drag laws (linear/quadratic) to produce more
energy loss after collision;
code still has bugs though
some clarifying comments - possible sign error?

absorb -> -absorb? or is this an energy input instead of a loss?