Merged Data format and Data sturcture sections into one. Minor edits in share your data section

These edits represent my attempt to decrease the amount of redundancy between the data formatting and data structure sections (i.e., the 4th and 5th sections).  If you find this merge to be successful then we'll have to renumber the remaining guidelines.  The primary motivation in part for making this merge was that the specific recommendations on file content formatting only appeared to apply to tabular data. Data with clearly defined standards typically have their own special file format and the specific structure and file content recommendations don't really apply in those cases.