Add save_capture_as_bands() method. Add out_data_type parameter.

[and-viceversa]

The scope of this PR was a little broader than expected.

The unanticipated challenge when allowing multiple output data types is that handling the img_type is None case creates too many logical branches and the standard if else clutter for no real benefit. The simplest thing to do was change img_type from a kwarg to a positional argument. Forcing the choice works because there are only 2 potential outputs, and handling the None case with available data just felt like it added confusion especially in the higher level ImageSet.

This PR:

• Change Capture.create_aligned_capture() to accept img_type as a positional argument. Simplify radiance/reflectance logic.
• Adds Capture.save_capture_as_bands() to meet the original intent. This includes output data type handling, and simplified logic depending on whether user passes 'radiance' or 'reflectance' value.
• Update `Capture.save_capture_as_stack() to also accept the new output data type handling.
• Update ImageSet to accomodate output data type parameter. Also update img_type to positional argument to match Capture usage. Some docstring improvements.

[and-viceversa]

This may close #121 and the final requirements for #77.