Avoid the ambiguous term "color map"

Instead, use

• "heat map" for plots of color-coded z vs Cartesian x,y;
• "color scheme" for a color coding (called "color gradient" in Matplotlib)
• "color legend" for a bar that shows color vs z

References:

• https://en.wikipedia.org/wiki/Heat_map
• https://www.nature.com/articles/nmeth.1902
• https://mathworld.wolfram.com/ContourPlot.html confounds true contour maps and heat maps, which is not helpful

Change terminology in:

• GUI
• Documentation
• Python API and example scripts (including comments)