adding to/from Penman/Graph/Tree convenience methods

[chanind]

The transformations between penman strings / graphs / trees have non-obvious names, so this PR adds aliases to the Tree and Graph classes to make these transformations more obvious. These include:

• Graph.fromPenman(): equivalent of decode()
• graph.toPenman(): equivalent of encode()
• graph.toTree(): equivalent of configure()
• Tree.fromPenman(): equivalent of parse()
• tree.toPenman(): equivalent of format()
• tree.toGraph(): equivalent of interpret()