Changed the 'clam' output columns.
Merge branch 'master' of https://github.com/SimonGoring/neotoma
A number of changes based on comments from package collaborators.

* Fixed typo in `get_contact` and added `length`  as `fun.aggregate` for the contacts so that a warning isn't thrown.  It doesn't do anything anyway.
* `get_dataset` was changed to provide a `Site` component that lines up with the one in `get_site`.
* `get_site` now allows a dataset to be passed so that a set of datasets can be converted easily into sites.  We should do the same for downloads if it's not already like that.
* `get_download` now accepts datasets and pulls out the DatasetID so that we don't have to do that annoying `lapply`/`sapply` thing anymore.