Join On with all group by dimensions should work

Nobody has claimed this yet.

Given a script like:

``` go
var net_in = stream
|from()
.database('metrics24h')
.retentionPolicy('default')
.measurement('net_in')
.groupBy('object_id')

var net_out = stream
|from()
.database('metrics24h')
.retentionPolicy('default')
.measurement('net_out')
.groupBy('object_id')

net_in
|join(net_out)
.as('net_in', 'net_out')
.on('object_id')
.streamName('net_total')
|eval(lambda: "net_in.value" + "net_out.value")
.as('value')
|influxDBOut()
.database('metrics24h')
.measurement('net_total')
```

The join `on` should not change the default behavior of joining by group since it specifies the same number of dimensions as the group by dimensions above.

Start by reproducing the provided Kapacitor script, focusing on the groupBy dimensions and the join(...).on('object_id') behavior. Trace the join entry point and compare its grouping with the expected default behavior; done means joins with matching group-by and on dimensions preserve the group join behavior.

Written by the indexing model from the issue text.