move default alloc_specs from libE() definition to alloc_funcs.defaults

[jlnav]

Possibly addresses #325

This simple approach tries to address the concerns laid out in the above issue where the default alloc_specs were defined in the libE function definition.

This moves the default alloc_specs to /alloc_funcs. They are imported in libE.py, and can be imported and adjusted within calling scripts through:

from libensemble.alloc_funcs import defaults
alloc_specs = defaults.alloc_specs

Potentially, other default settings throughout typical libE projects can be imported and adjusted in this manner. Or, the default alloc_specs could be returned by parse_args().

[jmlarson1]

Looks good to me.

[coveralls]

Pull Request Test Coverage Report for Build 2679

• 5 of 5 (100.0%) changed or added relevant lines in 2 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.03%) to 94.646%

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Totals
Change from base Build 2673:	-0.03%
Covered Lines:	3997
Relevant Lines:	4185

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💛 - Coveralls

[shuds13]

I happy to go with this. Can you make sure its covered in docs/ - esp. how the user can import the defaults and modify.