Changes made in proofs

Changes include responses to last minute comments by several folks
on both GitHub and PeerJ.

1. Change standard file format examples to those that include
   better metadata in the file.
2. Mention that metadata can be included in data files.
3. Note that part of the reason for recommending against the last
   4 sets of null values is that they are uncommon.
4. Clarify what we mean to No License and Variable Access.
5. Standard typos and language cleanup.