Add lines for the density of infected individuals for the Garki model

Usage

xde_lines_X_garki(XH, pars, clrs = viridisLite::turbo(7), llty = 1)

XH: a list with the outputs of parse_deout_X_SIS
pars: a list that defines an exDE model (e.g., generated by xde_setup())
clrs: a vector of colors
llty: an integer (or integers) to set the lty for plotting