Added new functionality to the package to remove any number of Individuals from a Species (`Model.remove_individuals`), and to add any number of Individuals to a Species (`Model.add_individuals`) by either pulling them from another Species object or simulating them with msprime (`gnx.sim_msprime_individuals`). Along with this, also added a new, optional section to the species 'init' section of the parameters file (keyed to 'msprime') which provides parameters for simulating any number of Individuals from any number of msprime populations and placing them at specified coordinates on the Landscape (replacing the gnx-simulated starting population). (Also added a convenience method, `Model.burn`, to burn in a Model, because I was getting annoyed having to type something like `Model.walk(10000, 'burn')`.)