Add a "verified" column to data tables.

[uvchik]

This issue came up because the windpowerlib imported a corrupt data set from the OEP. (see #100)

A column that shows the review state could help such libraries to import data from the OEP. Such libraries could filter non-verified data from the data set to avoid problems.

We could discuss whether we want a True/False solution or a more detailed one and how we would like to name such a column.