New VariableTree syntax -- API INCOMPATIBILITY

All VariableTrees must now be 'wrapped' by a VarTree, not a Slot.
Initialization is now handled so that you don't have to add the tree in __init__().
Existing code will be broken, but errors should be reported that state to use VarTree.

Published plugins (analysis_server_plugin, drea_wrapper, flops_wrapper, hsrnoise_wrapper, overflow_wrapper) have been updated, but no pull issued pending knowledge of the correct framework version to require.