Patch_sampling possible w/ mask_level > max_level

[trislaz]

When the level of the mask is set to be greater than the number of
levels of the slide (max_level), then get_whole_image immediately adapt
the level to slide.level_count - 1. This commit updates level for the
rest of the procedure in Patch_sampling, otherwise we would get an
error.

[PeterJackNaylor]

Hey,

That is a good idea. Maybe, we could implement something slightly different:
when the resolution is not possible, we are asking a level that is too high, we could just take the lowest resolution of the image (with true_level for instance), and downsample it to the asked resolution.

This could have extra implication, and specifically for the fonctions in utils.py, like get_x_y, get_x_y_from_0, get_size that would have to deal with this too.
How does that sound? do you see have necessary changes? I believe that if we change those functions, all the functions in patch_sampling should remain the same.