Sample a predefined number of generations in SCEUA

[lu-kas]

This PR covers two things in the SCEUA algorithm:

1. It corrects the breakpoint logic, as the breakpoint was written too often.
2. It adds the possibility to define a number of generation to be computed during sampling.

[coveralls]

Pull Request Test Coverage Report for Build 849

• 10 of 22 (45.45%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.2%) to 89.774%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
spotpy/algorithms/sceua.py	10	22	45.45%
<!--	Total:	10	22	45.45%	-->

Totals
Change from base Build 843:	-0.2%
Covered Lines:	4135
Relevant Lines:	4606

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💛 - Coveralls

[thouska]

Great thank you for contribution, this feature will be available in Spotpy version >1.5.11 on PyPi

[lu-kas]

Thanks for your great tool! If issues arise in the context of my contribution, please assign me to them directly.