Make desiutil optional in setup.py

[weaverba137]

This PR supplements #54 by making desiutil not required for the operations of the top-level setup.py file.

@armengau, I also want to update the change log based on changes recently made directly to the master branch. Could you please provide some text for that?

[coveralls]

Pull Request Test Coverage Report for Build 469466906

• 0 of 9 (0.0%) changed or added relevant lines in 1 file are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage remained the same at 1.876%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
py/prospect/utilities.py	0	9	0.0%
<!--	Total:	0	9	0.0%	-->

Files with Coverage Reduction	New Missed Lines	%
py/prospect/utilities.py	1	13.45%
<!--	Total:	1		-->

Totals
Change from base Build 467134825:	0.0%
Covered Lines:	59
Relevant Lines:	3145

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💛 - Coveralls

[armengau]

@weaverba137 Very sorry for having done those commits within the master those last days. For the change log:

• Adapt scripts to blanc ("deep" directories)
• Support sv1_bgs_mask

[weaverba137]

@armengau, thank you, that should be all I need.