Clip day and night composites after enhancement

[pnuu]

This PR fixes the DayNightCompositor for channels with fixed stretching for which can have values exceeding 1.0 after enhancement. Those larger-than-assumed values will cause problems when blending (solar zenith angle weighed average) with another channel.

• Tests passed
• Passes git diff origin/develop **/*py | flake8 --diff

[coveralls]

Coverage remained the same at 60.581% when pulling e5f832b on bugfix-DNC into 985cce1 on maint/0.8.

[coveralls]

Coverage remained the same at 60.581% when pulling e5f832b on bugfix-DNC into 985cce1 on maint/0.8.

[coveralls]

Coverage remained the same at 60.581% when pulling e5f832b on bugfix-DNC into 985cce1 on maint/0.8.

[pnuu]

The flake8 test outputs these (unrelated to my little fix) warnings after suppressing the default maximum line length warnings:

$ git diff origin/develop **/*py | flake8 --diff --max-line-length=120
satpy/__init__.py:7:15: W291 trailing whitespace
satpy/__init__.py:11:1: W293 blank line contains whitespace
satpy/__init__.py:32:1: F401 'satpy.version.__version__' imported but unused
satpy/__init__.py:33:1: F401 'satpy.dataset.Dataset' imported but unused
satpy/__init__.py:33:1: F401 'satpy.dataset.DATASET_KEYS' imported but unused
satpy/__init__.py:33:1: F401 'satpy.dataset.DatasetID' imported but unused
satpy/__init__.py:34:1: F401 'satpy.readers.DatasetDict' imported but unused
satpy/__init__.py:34:1: F401 'satpy.readers.find_files_and_readers' imported but unused
satpy/dataset.py:121:1: E305 expected 2 blank lines after class or function definition, found 1
satpy/scene.py:32:1: F401 'satpy.config.runtime_import' imported but unused
satpy/scene.py:279:9: F841 local variable 'unknowns' is assigned to but never used
satpy/utils.py:42:1: E722 do not use bare except'