Automate assertions of expression to grouping nodes, based on leaf node assertions

[dosumis]

Input: Assertions of marker genes expression linked to (some subset of) leaf nodes. For any ancestor node that groups only nodes with maker gene assertion, add the same marker gene assertion to that ancestor node.

Currently the relevant assertions are in a ROBOT template. We should probably split these out into a simpler, separate input table. Implementation is deferred until this has been done.