Fix channel_axis bug in transforms.py

[nickeener]

Found a bug while running the command line version of the tool. If you give it a multi-z multi-channel image with a dimension order of (z, ch, y, x) (as suggested in the documentation) and correctly set the --z_axis parameter to 0 and the --channel_axis parameter to 1, this chunk of code in convert_image() of transforms.py:

# put z axis first
if z_axis is not None and x.ndim > 2:
    x = move_axis(x, m_axis=z_axis, first=True)
    if channel_axis is not None:
        channel_axis += 1
    if x.ndim==3:
        x = x[...,np.newaxis]

shouldn't change the image at all since the z_axis is already first but it will still add 1 to channel_axis, making it wrong because the z_axis wasn't moved. Adding an additional check before running this chunk to make sure the z_axis needs to be moved fixes this.