Use sample ID in place of label for sample queries

This updates the sample DataField instance to map to samples.sample.id
and defines the label_field_name as 'label' so visually the user sees
the sample label while queries use the ID for its speed benefits.

Signed-off-by: Don Naegely <naegelyd@gmail.com>
Merge pull request #182 from cbmi/issue-181

Use sample ID in place of label for sample queries