Fix crude stretch not calculating min/max per-band

[djhoese]

Doing data_arr.min() computes the min/max across all bands together. We want this separated out so we end up with a minimum value for each band separately.

• 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 (+0.04%) to 86.925% when pulling 359a730 on djhoese:bugfix-crude-minmax into c89ba18 on pytroll:master.