warning when user clips from a valid extent, but no points remain

[bw4sz]

Seems sensible. Up to you if you want to throw an Exception. The silent behavior here tripped me up. I pulled from master.

[coveralls]

Pull Request Test Coverage Report for Build 265

• 3 of 3 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.09%) to 75.32%

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Totals
Change from base Build 263:	0.09%
Covered Lines:	647
Relevant Lines:	859

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💛 - Coveralls

[brycefrank]

I like it. Thanks for the PR, will merge to master for now. I think a warning is better than an exception myself.