Figure.colorbar: Add alias 'dpi' for the -N option

[seisman]

This PR adds an alias for the -N option (xref: https://docs.generic-mapping-tools.org/6.6/colorbar.html#n)

-N[p|dpi]
Controls how the color scale should be encoded graphically. To preferentially draw color rectangles (e.g., for discrete colors), append p. Otherwise we will preferentially draw images (e.g., for continuous colors). Optionally append effective dots-per-inch for rasterization of color scales [600].

This option has four possible use cases:

• No -N: GMT defaults to drawing the color scale as images or rectangles depending on different conditions (e.g., a continuous or discrete scale).
• -Np: draw color rectangles
• -N300: draw as image with a dpi of 300
• -N: equivalent to -N600

This PR provides the dpi parameter:

• dpi=300 -> -N300
• dpi=0 -> -Np (a zero dpi makes no sense so it's mapped to -Np)

Notes:

• dpi=True is not supported to make the parameter simple.