make engines specify the view type

now the data dumped by test.test_vtk_explorer is useable by
the cinema web app
add in a working paraview explorer

todo:
fully specify slice options
figure out how best to specify range of LUTs for each slice
figure out how to mix camera, color and operation explorers
flesh out the chaining concept for parameter sets

the appraoch is to give the explorer multiple engines with
well defined order to call them in
Merge pull request #7 from demarle/add-engine-infrastructure

Add engine infrastructure