Episode 4, Exercise 4 Visu with matplotlib

### Brief description

1. Give examples for basic `imshow` functionality
2. Introduce named/built-in matplotlib [colormaps](https://matplotlib.org/stable/users/explain/colors/colormaps.html) and `vmin, vmax` arguments and discuss their properties and requiremnts in terms:
- contrast
- relation to the input channels' wavelength
- accessiblity and perceptual uniformity (color blindness)
- mixing/blending with other colormaps (see Episode 4 - Visualization with Napari)
3. Basic subploting with matplotlib and most relevant Axes functions `set_title`, `set_xlim`, `set_axes_off` etc
4. Add colorbar to `imshow` plot + basic positioning

### Learning objective(s)
[covered](https://github.com/carpentries-incubator/bioimage-analysis-python/wiki/Learning-Objectives#visualization-with-matplotlib)

### Volunteer(s)
@sommerc

No lesson file or test is named in the issue; first locate the Episode 4 exercise source in the repository and review the linked matplotlib colormaps documentation. Done should cover imshow examples, colormaps with vmin/vmax, subplot and Axes usage, colorbars, and the listed contrast, wavelength, accessibility, and blending topics.

Written by the indexing model from the issue text.