#17: delete coordinate_validation_status, coordinate_validation_date, and coordinate_validator_id from fixtures for pytest

#17 and #9: stationmeta patch API writes (one) entry to changelog table; schema shows changelog table when requesting stationmeta from database

#11 and #3: added more readable REST API for controlled_vocabulary (and also for requests of only one entry))

#4: bug fix done for nested stationmeta (inside timeseries record); still left to do: clean up of code and adapting pytests

#11 and #3 (and also #8): provided an API to the controlled vocabulary (as long as no ontology service is in place), also wrote first pytest, but not yet working (settings have to be established in test database), prepared scripts to create pages also documenting the controlled vocabulary (pages: index can still show the source code, which is wanted)

#11: still struggling with creating pdf (but now a little bit better) -- see (pandoc) command in issue

#11: started adding descriptive text for automatic openapi documentation (especially for use documenting metadata)

#3: found solution for loading controlled vocabulary only once at start time (with raw database connection); have to put it into place (so that pytests are also running); for now the workaround is still active

#4: nested download for station inside timeseries still not working properly, but at least shows some information; upload after gitlab crash

#4: nested download was extended for timeseries; TBD: showing nested station information is not working yet; also changed programme from many-to-many to 1:n

#3: added unique constraint to controlled vocabularies; added foreign keys (controlled vocabularies) to models

#3,#4: nested uploads/downloads using controlled vocabulary; still to investigate: how to load enum_tables at beginning (this commit is a workaround)

#3: now nested human readable download is established for timeseries; TBD: do this for all other modesl

#3: toar_controlled_vocabulary is used for downloading timeseries, pytests are added for timeseries; not yet working: how to show human readable fields inside nested models (how is this 'conversion' done automatically?)

almost done: using human readable controlled vocabulary for upload; TBD: using human readable controlled vocabulary for download, adapting pytests

#7: default values for stationmeta_global were defined; #4: nested upload tested for 'stationmeta_global' (not defining all metadata -- getting the rest from default values)

#4: nested upload for stations done; TBD: more testing (esp. pytest), use controlled vocabulary (related to #3)

#4: nested download for stationmeta realized; TBD: uploading of new test database, nested download for stationmeta