Refactor render channels to render images

[sagar87]

• Basic implementation of render_channels.

[sagar87]

Complements pl.render_images with the ability to plot specific channels, i.e. (shown is the steinbock_io example from the sandbox)

fig, ax = plt.subplots(2, 2, figsize=(9, 8))

# render channel 1
stein.pp.get_elements('Patient4_005').pl.render_images(channel=[1]).pl.show(ax=ax[0, 0]) 
# render channel 1 and overlay segmentation mask
stein.pp.get_elements('Patient4_005').pl.render_images(channel=[1]).pl.render_labels(fill_alpha=0.).pl.show(ax=ax[0, 1]) 
# plot channel 1, 2, 3 and use default color palette
stein.pp.get_elements('Patient4_005').pl.render_images(channel=[1, 2, 3]).pl.show(ax=ax[1, 0]) 
# plot channel 1, 2, 3 and let user define the colors
stein.pp.get_elements('Patient4_005').pl.render_images(channel=[1, 2, 3], palette=['seagreen', 'yellow', 'red']).pl.show(ax=ax[1, 1])