Emergency patch to fix #190

Replace reshape2::dcast with data.table::dcadt for speed gain and to avoid issue with long sample names causing vector return error in reshape2 but not data.table:

Using reshape2:

```
 obs_counts <- dcast(obj$obs_norm, target_id ~ sample, value.var = value_name)
Aggregation function missing: defaulting to length
Error during wrapup: dims [product 868328] do not match the length of object [41674225]
```

But no error using data.table.

[`obj_norm`](https://s3.amazonaws.com/rech-ul/obj_norm.RDS?AWSAccessKeyId=AKIAI6SXE4VOIPIZJI6Q&Expires=1601162547&Signature=3lPIt6yZ2UPFhMioUwwRopzo8eM%3D) to reproduce:

For d05cd9b ed95e7

If `tabd_df` is not a data table, it is converted. Don't need `as.data.table`.