This method places a cell in the tissue.
Examples
# set the seed of the random number generator
set.seed(0)
# create a tissue simulation
sim <- SpatialSimulation()
sim$add_mutant(name = "A",
epigenetic_rates = c("+-" = 0.01, "-+" = 0.01),
growth_rates = c("+" = 0.2, "-" = 0.08),
death_rates = c("+" = 0.1, "-" = 0.01))
# add into the tissue a cell of species "A+" in position (500,500)
sim$place_cell("A+", 500, 500)