add write_vtk to PorousMaterials.jl expose it

[SimonEnsemble]

• expose write_vtk
• remove old function write_unitcell_boundary_vtk

(must hv forgotten to push this to the pull request a few days ago, sorry. but this is a good test to see if the Travis CI info pops up!)

[SimonEnsemble]

@ahyork w00t, Travis CI is showing its tests! :+1:

looks like as long as the travis.yml file is included in the pull request, the integration tests will run.

still the badge is missing from the README.md though.

[coveralls]

Pull Request Test Coverage Report for Build 13

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 56.113%

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Totals
Change from base Build 9:	0.0%
Covered Lines:	716
Relevant Lines:	1276

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💛 - Coveralls