on_before_epochs image grid using icplot

Copied from James's comments on #8 (closed):

For the on_before_epochs case a type of plot we probably want is a grid of images from a sample from the training set.

icplot has low-level building blocks for this - like the ImageSeries, but it isn't set up to do it from the higher level calls, like 'generator.plot()'. The first step is to update icplot so that it can take a list of images in generator.plot and through the passed in PlotConfig can make a plot of these images. So this could first be a simple unit test in icplot.

The main work is here: https://git.ichec.ie/performance/toolshed/icplot/-/blob/main/src/icplot/graph/generator.py?ref_type=heads#L22 It can have an 'if' check for the Series type and if it is a grid of images then have some custom handling.