"shape" in agent_portrayal corresponds to "marker" in matplotlib's scatter function.

Previously, if only some agent_portrayals provided info like size, color, matplotlib would not accept the parameters and the program would crash.
Additionally, edited some names for coherence.

error message shows public names of visualization attributes.
default color and marker fixed to "tab:blue" & "o" respectively

for more information, see https://pre-commit.ci

Viz tutorial now mentions that in addition to size and color, default drawer allows for custom shape.