Avoid adding of a batch dim on 2d signals.

[felixblanke]

For 1d signals the coefficients as well as reconstructed signals keep the same number of dimensions as the original input signal.
However, for 2d signals with only 2 axes we currently always add a superfluous batch dimension, i.e. [H, W] -> [1, H, W].

This seems inconsistent.

We can address this by introducing a functionality similiar to ptwt.conv_transform._postprocess_result_list_dec1d for higher dimensions.

[v0lta]

#93 is merged.