[simplify] fix double, replaced by template parameter
[simplify] prepare test for distance object instead of always value, and
added overload accepting a strategy
[simplify] prepare for other simplify strategy: added testcase
[simplify][test] added test with strategy
[simplify][test] splitted tests such that test with strategy does not
test the version without strategy, to avoid mixing strategy types
[simplify] prepare for Adam's enhanced douglas_peucker with pull request #89