Make study datatype selection checkboxes

This changes from the selection of specific processed data for a study and data-type to a checkbox for the datatype of a study. We are of course assuming that we have only a single processed data object for a datatype in a study, but since we are only processing with the default pipeline currently this should be a safe assumption. We will need to revisit this later though, and issue #215 is still open for it.

I also fixed a small bug with the javascript verification for removing samples in an analysis. 
A Cartesian Product where the first list is of size 1 is the same as looping over the other list...:

```python
proc_data_id = int(proc_data_id)
for study_id in self.get_arguments(study_id):
    yield (proc_data_id, study_id)
```