Improvement: Consider adding a less verbose method for a single constraint

Consider this code - when there is only one constraint, wrapping each element in datastructures seems unnecessarily verbose.

```r
setConstraints(
paths = c("Gene"),
operators = c("IN"),
values = list("PL_Pax6_Targets")
)
```

**Suggestion**
It would be nice to either overridethe method signature for `setConstraints` to allow something like this

```r
setConstraints(
paths = "Gene",
operators = "IN",
values = "PL_Pax6_Targets"
)
```

or alternatively, if this isn't something that can be done in R, maybe just adding the method `setConstraint` (note no `s` at the end) would achieve the same thing.

This is low priority but would probably be easy to implement while fixing more important constraint issues such as #58