export images from time steps

[coveralls]

Pull Request Test Coverage Report for Build 1953878330

• 84 of 210 (40.0%) changed or added relevant lines in 10 files are covered.
• 4 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-3.1%) to 74.093%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
honeybee_vtk/actor.py	2	3	66.67%
honeybee_vtk/scene.py	5	9	55.56%
honeybee_vtk/assistant.py	10	15	66.67%
honeybee_vtk/cli/export.py	5	12	41.67%
honeybee_vtk/filter.py	4	16	25.0%
honeybee_vtk/model.py	13	34	38.24%
honeybee_vtk/timestep_images.py	27	103	26.21%
<!--	Total:	84	210	40.0%	-->

Files with Coverage Reduction	New Missed Lines	%
honeybee_vtk/actor.py	1	78.65%
honeybee_vtk/cli/export.py	1	36.96%
honeybee_vtk/model.py	1	77.06%
honeybee_vtk/scene.py	1	73.74%
<!--	Total:	4		-->

Totals
Change from base Build 1953869467:	-3.1%
Covered Lines:	1716
Relevant Lines:	2316

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💛 - Coveralls