Geonomics

Debugged Model.plot_density().

Fixed the nlmpy error I had created by moving the 'module not avail' error inside the only function that uses nlmpy but forgetting to move the import statement there too.

Fixes a bug in species._plot_movement_or_dispersal that accidentally set movement_distr_param2 to the value of movement_distr_param1

Added plot_movement() and plot_dispersal() methods to the Model object, to help with visual design and debugging of sims.

Fixes some minor bugs and better handles when `Species._remove_individuals()` does and does not print out information.

Debugged so that gnx can now row models with rectangular landscapes (checked by plotting [genetic PCA for randomly generated landscape](https://github.com/erthward/geonomics/assets/6105510/61cf9941-a7e1-43ad-ae8a-ce01723fc236), plotting [genetic PCA for a raster-loaded landscape with an msprime-initiated population](https://github.com/erthward/geonomics/assets/6105510/f259e942-6515-43b8-a799-8a735f937a6d), and plotting [phenotypes for raster-loaded landscape with selection](https://github.com/erthward/geonomics/assets/6105510/2e76de1f-0bb5-4a52-95ef-472550e206fa)). Also debugged burn-in for msprime-initiated species, which were still needlessly running one burn-in timestep before dumping and replacing individuals. Also made some other minor tweaks along the way (see commits).