solvers: space output better.
solvers: add and use root_solver_message(); add colours to report Xlow,Xhigh.
planets/planet_position(): print comet name if light time diverges.
comets: print convergence warnings in yellow.
system/print_run/cpu_time(s)(): use grey text for screen output.
date_time/print_date_time_and_location(): print grey text to screen.
io: define ANSI colours as constants, then use them.
io: make red() etc. true wrappers for clr(); add grey.