FIX: fix sql syntax error; fixes #23

Closes #23

See merge request caosdb/caosdb-mysqlbackend!21

This adds two missing branches in the `getDateWhereClause` function and fixes an consistency in the `constructDateTimeWhereClauseForColumn` function.

# Focus

1. Don't let the whitespace changes confuse you.
2. The changes in `constructDateTimeWhereClauseForColumn` merely add an "OR value_ns IS NULL".
3. The changes in `getDataWhereClause` add an ELSEIF branch for each operator.

WIP ENH: Documentation

See merge request caosdb/caosdb-mysqlbackend!20

Quazgar authored 4 years ago and Henrik tom Wörden committed 4 years ago

Also amended Changelog and Readme.

EHN: version history

See merge request caosdb/caosdb-mysqlbackend!19

f-query-version -> dev

See merge request caosdb/caosdb-mysqlbackend!18