Add more control for building trees

[allenbaron]

More fine grained control is needed when building trees. It's not clear if this should be handled programmatically or after careful exploration and decisions by the user but some improvements for flexibility are probably needed.

Add control of included subclasses

DO.util's *_subtree() commands have only a top_node argument and build a subtree including all terms below that superclass. When a top_node is in the upper portion of the tree, the number of children can be overwhelming and unnecessary for creating text tree diagrams.

This might be solved by specifying the children of interest and finding the lowest common subsumer programmatically but that might not provide enough control.

Another approach might be to add something similar to ROBOT extract's MIREOT lower-terms argument.

Add ability to create multiple trees

More than once the need for multiple subtrees have been necessary to avoid including a lot of superfluous terms. The ability to create multiple trees with a single command might be helpful. It might also be helpful to allow specifying a maximum distance between classes or a maximum number of total classes to limit tree output?