Fix np.float16 to string conversion

[cheginit]

Based on this bug report in numpy's repo, rounding a np.float16 could lead to inf. So instead, I propose to use a numpy function for converting a float to str. For example:

np.format_float_positional(np.float16(7.33), 4)

correctly return 7.33 but:

str(round(np.float16(7.33), 4))

returns inf.

[coveralls]

Pull Request Test Coverage Report for Build 904

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• 2 unchanged lines in 1 file lost coverage.
• Overall coverage increased (+0.1%) to 84.693%

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Files with Coverage Reduction	New Missed Lines	%
spotpy/algorithms/sceua.py	2	85.46%
<!--	Total:	2		-->

Totals
Change from base Build 903:	0.1%
Covered Lines:	4111
Relevant Lines:	4854

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💛 - Coveralls