Added spatially varying external pressure through the new external_field argument to kinetic_ising().
Added support for spatial masks: NA cells are excluded from the simulation and no longer contribute neighbor pressure.
Added a vignette showing how probability maps can guide kinetic Ising simulations.
Bug fixes
Cell locations are now sampled uniformly during simulation. Previously, cells on the outer rows and columns were selected less often than interior cells.
composition_index() now ignores masked cells consistently for matrices and rasters.