Add option for preserving dimensions of length 1

[tlnagy]

Currently, we drop all dimensions that have a length of 1:

OMETIFF.jl/src/loader.jl

Lines 150 to 152 in 4adee1a

	unused_dims = Tuple(idx for (idx, key) in enumerate(keys(dims)) if dims[idx] == 1)
	squeezed_data = dropdims(data; dims=unused_dims)
	used_axes = [axes[i] for i in 1:length(axes) if !(i in unused_dims)]

since I originally considered them superfluous, but there could be circumstances when having that information would actually be beneficial, e.g. for saving (see #30) or for verification.