added function clean_world, and beta and alpha diversity ( to be continued). Modified Plots.plot, mainly for integrating the geotrait into option "x" and "xs"

version 0.1.8, with new documentation, old competition and birth function deleted because now passed by the used.

Changes on type Agent, that is now Agent{T,U}. Added a function increment! for type Agent{MixedAgent,U}.

Introducing new abstract type to distinguish between different types of agents : continuous, discrete or mixed.

Test have also been modified, because it seems that this change also changes the random seed.

Agent file: work in progress, we should remove competition kernel and carrying capacity
Gillepsie: modified updates to use kernel that we now pass as arguments through the parameter dictionary p
runworld: for now we only modified std update but this should be done for all the project