Error handling for empty requests

Originally slices where the start equals the stop or the start exceeds the extents then an IndexError
is raised through the stack resulting in problems reading from the coverage. This commit adds error handling in the
get_value_dict logic to return empty arrays in these two cases instead of raising an error.
Merge pull request #135 from ooici/empties

OOIION-1502 Error handling for empty requests