various improvements to analysis demo

1.  Remove hardcoded "SVL" early-burst model parameter.
    Instead, we now populate a list of the character matrix
    headers & ask the user to pick which one should be used for
    model fitting.
2.  Loosen restrictions of vtkTreeHeatmap visualization.  It can
    now be used to view a dendrogram or a heatmap in isolation.
Merge branch 'master' of https://github.com/arborworkflows/ArborWebApps