Always write and/or require dc:description for non-standard CLDF columns

[chrzyki]

Whenever a table is non-standard CLDF (no propertyUrl pointing to terms in any version of the CLDF ontology), it might be beneficial to always write and/or require a dc:description with pylexibank. See for example the CLDF version of APiCS for a number of non-standard CLDF columns with good dc:descriptions.

[xrotwang]

While this would be probably good practice, it might be overkill to enforce it - and will make re-creating old datasets with new pylexibank versions more fragile. Thus, I'd rather add this to recommendations somewhere.