rename config option grain_crop_plot_size_nm

[ns-rse]

We have an option in the plotting configuration grain_crop_plot_size_nm. If this value is -1 then grains are plotted
as is, otherwise it is used as the argument target_size_nm in the function
re_crop_grain_image_and_mask_to_set_size_nm.

It would be useful if we considered renaming configuration options to align with the parameters of functions they are
used in (this allows the convenience of passing **kwargs to the functions).

Possible options to rename to...

• target_size_nm the simplest rename.
• min_grain_crop_plot_size_nm a more descriptive name (I'm assuming the cropping is a minimum value though), but this
  would require updating the re_crop_grain_image_and_mask_to_set_size_nm() function too.