added DualScaleBar, etc.

[tait-py]

Thank you for your package, @ppinard! I use it extensively in my own work, and over the last few weeks have been making some changes to it for my own use, which often involves plotting time-series data with scale bars in both the x and y axes. I believe some of these could be useful to other users, so I wanted to share them with you for your consideration. Please see a full changelog below. I look forward to hearing your thoughts.

• added DualScaleBar class, which plots two aligned scale bars for both the x and y axes, and corresponding tests
• added Time dimension to support time-series data (representing time as [si_prefix]s, not minutes/hours/etc.), and created corresponding tests
• added the option to place a ScaleBar at a specific coordinate by passing an (x, y) coordinate to location, and updated tests and documentation accordingly
• updated documentation to describe how to set dx when plotting non-image data
• added new SI prefixes (as of 2022-11-22): ronna (R, 10^27), quetta (Q, 10^30), ronto (r, 10^−27) and quecto (q, 10^−30).
• fixed typo at scalebar(340): initialize -> initialiazed
• modified ValueError at scalebar(563) to be more descriptive: "Invalid unit ({unit}) with dimension" -> "Invalid unit ({units}) with dimension ({self._dimension.class.name})")