Update _algorithm.py

[florianjehn]

Last update of this file destroyed ability to use several objective functions, because of fixing a value that should be flexible.

[coveralls]

Pull Request Test Coverage Report for Build 804

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• 8 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.09%) to 89.917%

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Files with Coverage Reduction	New Missed Lines	%
spotpy/algorithms/_algorithm.py	1	86.72%
spotpy/algorithms/rope.py	1	93.58%
spotpy/algorithms/dream.py	6	89.22%

Totals
Change from base Build 801:	-0.09%
Covered Lines:	4129
Relevant Lines:	4592

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💛 - Coveralls