add data types to extra variables

[alex-cit]

fix #749

• data types added to Variables
• position variables added to a table (instead of a list) to be consistent across the docs

[tomasMizera]

Gpkg-like types

[alex-cit]

Data types are now used: Text (string), Number (integer), Number (decimal), Boolean, Geometry.

Redirects check fails due to new page, should not affect this PR.