optOrEnvVar retrieves that value assocaited with the
name provided as an argument, prioritizing in its search
by first interpreting the given argument as an R
option name, and then trying an interpretation as an
environment variable if and only if the R option
is not set or is an empty string or vector.
optOrEnvVar(name)The name of the option or env var to fetch
The value associated with the given name,
returning NULL if and only if the name is
set neither as an option nor as environment variable.