Version 0.2. Changed the structure of agents, to better integrate the geotrait. Changes have telescoped to accessor get_geo that now takes t as an argument, and all the functions that are using it.

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"

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