Re-enable the berea integration test and the generate voxel image test when skfmm is sorted out

[ma-sadeghi]

The issue with scikit-fmm seems to have been fixed (see here)

[jgostick]

• [x] re-add porespy to test_requirements
• [x] Uncomment this file: https://github.com/PMEAL/OpenPNM/blob/dev/tests/integration/PoreSpyIO_on_Berea.py
• [x] Uncomment this test: https://github.com/PMEAL/OpenPNM/blob/bc228c59e60376dd59fef4ecd505d13de67241e7/tests/unit/topotools/PlotToolsTest.py#L64