Added node_factory validation facility to the node library
Improved a comment
Improved a docstring
Implemented LiteralNode objects
Added a test for node.to_python()
Added examples for single-plane and multi-plane ruptures
Added examples of nonparametric ruptures
Heavily refactored LiteralNode and added support for line numbers in the error message
Fixed a docstring
Removed nonvalidated examples for the moment
Renamed to_python -> to_literal
Merge pull request #130 from gem/nonparametric

Added a "node_factory" validation facility to the node library