cpm2tm can fail with strange error when using inappropriate input

[rdiaz02]

A user might try this

d1 <- data.frame(From = c("Root", "Root", rep(c("A", "B"), 3)),
                              To = c("A", "B", rep(c("C", "D", "E"), c(2, 2, 2))),
                              Relation = rep(c("Single", "AND", "OR", "XOR"), rep(2, 4)),
                              Lambdas = c(0.7, 0.8, rep(c(0.9, 0.4, 0.5), c(2, 2, 2))))

evamtools:::cpm2tm(d1)

which fails as

Error in as.environment(where) : using 'as.environment(NULL)' is defunct
> traceback()
2: exists("Relation", x$edges)
1: evamtools:::cpm2tm(ex_XOR_AND_OR)

So we probably want to:

1. Check for the existance of the edges component and error early and with a clear message (like "No edges component in input. You are calling this function in an unintended way and/or with the wrong input").
2. Use "Relation" %in% colnames(x$edges) instead of exists("Relation", x$edges): the second fails when x$edges does not exist.