Overview

• fixes OccResponse in occurrences #171; there were some inconsistencies with data returned by search queries on occurrences. For backwards compatibiity, execute still returns a DataFrame object, but sets data in OccResponse as dict({total: int, results: DataFrame/dict}). This fixes failing to_pandas method.
• raise a NotImplementedError on calling to_pandas for non-supported data types
• Resolves some FutureWarning while concatenating DataFrame during search. NA value columns while being merged with non-NA ones was throwing this warning. We do the merges while paginating large queries as API supports only 10k records at a time. So, we merge these chunks into one. Resolution was to skip merge during the first one (when the original results DataFrame is empty).
• Add corresponding unittests.
• Tiny typo on README that introduced an annoying tilda (`) on the link. Possibly came up during conversion from rst -> md.

Thanks.