Added interpolations function in utils.jl. Minor change on world2df, with argument `geotrait=false added`. Minor change on plot

added option "gs" for plotting, adding a gradient of colour for "3dgeo" and "3d" option in plotting, added arithmetic and geometric smoothing

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"

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

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.