Fix casting and scaling float arrays in finalize

[djhoese]

Yet another bug found. If you use finalize on an RGBA array represented by floats it wasn't casting it to the output data type which was causing an error by rasterio. Additionally, it wasn't being scaled because it already had an alpha channel.

This should all be fixed now so all float input gets scaled to the output data range regardless of mode and is assumed to be between 0 and 1. All integer input data is left as is and is assumed to be the expected values. Any integer values that don't fit in the output data type are clipped. I've tried adding all this information to the docstring for finalize as well.

• Tests added (for all bug fixes or enhancements)
• Tests passed (for all non-documentation changes)
• Passes git diff origin/master **/*py | flake8 --diff (remove if you did not edit any Python files)
• Fully documented (remove if this change should not be visible to users, e.g., if it is an internal clean-up, or if this is part of a larger project that will be documented later)

[coveralls]

Coverage increased (+0.1%) to 88.394% when pulling 97d2491 on djhoese:bugfix-rgba-floats into b9ca0e7 on pytroll:master.