Add support for ONE OF constraints

`ONE OF` constraints aren't handled correctly by InterMineR and convert to invalid query XML.

```r
pancreasConstraint1 = setConstraints(
paths = c("Gene", "Gene.proteinAtlasExpression.level", "Gene.proteinAtlasExpression.tissue.name"),
operators = c("IN", "ONE OF", "="),
values = list("PL_DiabetesGenes", c("Medium", "High"), "Pancreas")
)
```

This constraint will return a 400 when run, because it's creating the following XML:

```xml

```

But it should be making something more like:

```xml

Medium
High



```