Cai-causal-graph

- Improved performance of checking for cycles when adding edges by avoiding repeated checks.

- Modified `cai_causal_graph.time_series_causal_graph.TimeSeriesCausalGraph.add_edge` in
`cai_causal_graph.time_series_causal_graph.TimeSeriesCausalGraph` to always order source and destination nodes by
`time_lag` even when the edge is not directed. For example,
`add_edge('c', 'a (lag=1)', edge_type=EdgeType.UNDIRECTED_EDGE)` will add an undirected edge `a (lag=1) -- c` instead
of `c -- a (lag=1)`. This is done just for convenience and to avoid confusion.

- Improved documentation.

- Added `cai_causal_graph.graph_components.Node.node_name` property to `cai_causal_graph.graph_components.Node` as an
alias of `cai_causal_graph.graph_components.Node.identifier`.
- Added `cai_causal_graph.causal_graph.CausalGraph.get_parent_nodes` and
`cai_causal_graph.causal_graph.CausalGraph.get_children_nodes` to `cai_causal_graph.causal_graph.CausalGraph`. These
return a list of the parent and children `cai_causal_graph.graph_components.Node` objects, respectively. This is to
supplement the `cai_causal_graph.causal_graph.CausalGraph.get_parents` and
`cai_causal_graph.causal_graph.CausalGraph.get_children` methods, which only return the node identifiers.

- Improved the `cai_causal_graph.time_series_causal_graph.TimeSeriesCausalGraph.get_topological_order` method for a
`cai_causal_graph.time_series_causal_graph.TimeSeriesCausalGraph` to better account for time.
- Fixed a bug in `cai_causal_graph.causal_graph.CausalGraph` and
`cai_causal_graph.time_series_causal_graph.TimeSeriesCausalGraph` that prevented
`cai_causal_graph.time_series_causal_graph.TimeSeriesCausalGraph.get_minimal_graph`,
`cai_causal_graph.time_series_causal_graph.TimeSeriesCausalGraph.get_summary_graph` and
`cai_causal_graph.time_series_causal_graph.TimeSeriesCausalGraph.extend_graph` from working properly as it did not
maintain the correct extra information such as node variable types.

- Added the `cai_causal_graph.time_series_causal_graph.TimeSeriesCausalGraph.get_nodes_at_lag` and
`cai_causal_graph.time_series_causal_graph.TimeSeriesCausalGraph.get_contemporaneous_nodes` methods to the
`cai_causal_graph.time_series_causal_graph.TimeSeriesCausalGraph` class to get the nodes at a given lag and the
contemporaneous nodes of the provided node, respectively.
- General improvements to several `from_*` methods in the
`cai_causal_graph.time_series_causal_graph.TimeSeriesCausalGraph` class.