formats _select_buildings() to allow vacancies by btype

[jessicacamacho]

Hi @janowicz !

To tackle the need for including building-type-specific vacancy rates (https://github.com/urbansim/consulting-planning/issues/159), I had to make a minor change to the _select_buildings( ) function in the develop module.

Thanks!

Jessica

[coveralls]

Pull Request Test Coverage Report for Build 42

• 5 of 9 (55.56%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.5%) to 95.284%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
developer/develop.py	5	9	55.56%
<!--	Total:	5	9	55.56%	-->

Totals
Change from base Build 36:	-0.5%
Covered Lines:	788
Relevant Lines:	827

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💛 - Coveralls