add export info metadata to dataset

[jenshnielsen]

This will eventually make it easier to export and reimport datasets

• Test for meta data being set, written and loaded correctly.
• A schema (dataclass -> str repr for this data)
• Let export write to this structure

[codecov]

Codecov Report

Merging #3135 (f94f1c2) into master (4ae08ff) will increase coverage by 0.04%.
The diff coverage is 97.50%.

@@            Coverage Diff             @@
##           master    #3135      +/-   ##
==========================================
+ Coverage   65.73%   65.77%   +0.04%     
==========================================
  Files         216      217       +1     
  Lines       28857    28895      +38     
==========================================
+ Hits        18968    19005      +37     
- Misses       9889     9890       +1