Add multiple features

[brittonsmith]

This adds the following new features:

• ds.rectangle_from_center function to specify a rectangular data container as center, width, height.
• I also noticed that we need to be transposing the data after reading from rasterio in order to get the x and y axes right.
• Switch coordinate system to absolute x and y positions instead of relative to left edge.
• properly flip axes to match data plotted directly from rasterio.
• add xpixels, ypixels, and pixels units