Added PlanetSize enum.
Added functions to return mass or radius in earth, neptune or jupiter multiples. Added function to determine fuzzy planet type based on radius or mass.
Now 'tweet' command uses the fuzzy planet size ascii art.
refactored format_planet_mass_str to use get_planet_size.
refactor rename
Updated ascii_system() so that tightly packed planets still appear by fudging the scale
Ascii system diagram now shows both pictogram for size and planet letter.
Added unicode Sun symbol for stellar mass
Fixed earth symbol, formatted masses better in summarize_system
Added a note