.data and .env pronouns

The .data and .env pronouns make it explicit where to look up attribute names when indexing V(g) or E(g), i.e. the vertex or edge sequence of a graph. These pronouns are inspired by .data and .env in rlang - thanks to Michał Bojanowski for bringing these to our attention.

The rules are simple:

• .data retrieves attributes from the graph whose vertex or edge sequence is being evaluated.

• .env retrieves variables from the calling environment.

Note that .data and .env are injected dynamically into the environment where the indexing expressions are evaluated; you cannot get access to these objects outside the context of an indexing expression. To avoid warnings printed by R CMD check when code containing .data and .env is checked, you can import .data and .env from igraph if needed. Alternatively, you can declare them explicitly with utils::globalVariables() to silence the warnings.