addTree functionality

[jto888]

Alessandro Veneri has requested an elegant method for adding fault trees generated separately into a given tree.

This is not as simple as he may have wished and I have identified two methods that will be pursued.

1. addTransfer could be a function used to import the resulting calculated event from the top of a source tree and entered as an undefined event in the target.
   Many packages utilize such transfer gate capabilities. These were mostly used to separate the graphics into chunks that could be displayed appropriately on one screen or printed output. For many years this was not implemented in FaultTree due to the ability to collapse gates selectively to enable presentation of several appropriate views of a large tree.

The addTransfer function is not expected to support live update as source tree(s) might be updated. It would be the user's responsibility to run the calculations on an updated subtree and re-run the script for the target in order to synchronize such transferred information.

2. ftree.combine would be a function that could truly add a source tree (from a defined by node, defaulting to top event) into a target tree at the node identified by the at argument.

The intention is that an entirely new tree script will be crafted. This requires that each of the source and target trees must be back-converted to script text permitting the text lines to be appropriately merged. Function ftree.combine will have to have a write_file argument, with optional dir for recording this new script, but I suppose it can be executed with do.call functionality.

There will be a number of qualification checks for each tree that may generate errors on execution with guidance for correction.