R: rename operators in V() and E()

_From @gaborcsardi on May 13, 2014 13:52_

To avoid situations like this:

```
library(igraph)
g <- graph.ring(10)
nei <- 1:10
V(g)[ nei == 1 ]
# Error in nei == 1 : 
#   comparison (1) is possible only for atomic and list types
```

If they start with a dot, then this is much less likely to happen.

_Copied from original issue: igraph/igraph#617_
