load_dataset duplicate check should use make_entry_id helper

I have undocumented support for an optional `Dataset` column for datasets in some places, but #90's change uses its own list of column names.  I should make that consistent by replacing this in `load_dataset`:

```
cts <- table(with(data, paste(Sample, Replicate, Locus, sep = "/")))
```

with this:

```
cts <- table(make_entry_id(data))
```
