Add essential IR tree operations

[dcerkoney]

Write unit tests and functions for essential tree transformations in the intermediate (computational graph) representation:

Transformations

Transformations for tree sanitization:

• prune_unary — Prune trivial unary operations, e.g., $(+a) \rightarrow a$ and $(*a) \equiv (1*a) \rightarrow a$.
• inplace_prod — Propagate subgraph factors up multiplicative chains and merge.
• merge_prefactors — Factorize multiplicative graph prefactors, e.g., $a * G + b * G \rightarrow (a + b) * G$.
• merge_chains — Merge operator chains into a single operation, e.g., $a+(+(\cdots+(+b)\cdots)) \rightarrow a+b$ and $G_1 +(+(\cdots+)+)\; G_2 \rightarrow G_1 + G_2$.
• merge_operators — Use the associativity of an operator $\mathcal{O}$ to "deparenthesize" an expression, i.e., join two sub-operations into a single $n$-ary form. Use with $\mathcal{O} = \bigotimes$ requires Base.:*(g1, g2).
• split_operator — Use the associativity of an operator $\mathcal{O}$ to "parenthesize" an expression, i.e., split the $n$-ary form of a node into two sub-operations. Use with $\mathcal{O} = \bigotimes$ requires Base.:/(g1, g2).

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High-priority transformations for the Parquet front-end:

• relabel — Update the label(s) of quantum operators in a graph and its subgraphs, e.g., {{1,2}=>3, 3=>4}.
• standarize_labels — Finds all labels in a graph and its subgraphs (e.g., 1,4,5,7...) and then relabels them in standard order 1,2,3,4,...
• unghost_legs — A special case of split_operator for graph decomposition $(\mathcal{O} = \bigotimes)$ which does not require Base.:/(g1, g2). Extracts a vertex with ghost legs and its connected propagators into a new $\bigotimes$ node, thereby "unghosting" its legs.
• replace_subgraph — Replace one subgraph of a graph g with another one of possibly different topology.

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Low-priority transformations requiring associative graph (de)composition to be defined via Base.:*(g1, g2) and Base.:/(g1, g2):

• expand — Expand a factorized graph expression: $(G_1 + G_2) * W \mapsto G_1 * W + G_2 * W$. The recursive depth should be specified by an argument depth which is 1 by default.
• factorize — Factorize a graph expression (the inverse of expand): $G_1 * W + G_2 * W \mapsto (G_1 + G_2) * W$. The recursive depth should be specified by an argument depth which is 1 by default. Requires Base.:/(g1, g2).
• tofeynmanrep — Convert a general graph to the Feynman representation by chaining expand and merge_chains.
• fromfeynmanrep — Optimize a graph in the Feynman representation by chaining factorize and merge_chains.

Definitions/Examples

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