Closes #62

The previous definition could result in unexpected errors.
The previous oneof(...) throws an instantiation error
when the parsed term is not ground. This may happen for example
when a JSON object is parsed to a dict (tag is a variable).
The replacement has the same semantical meaning but avoids
throwing an instantiation exception when applied on a dict.

This reverts commit f2bc75f5.

Restrict /bg/clear to POST requests

Closes #60

See merge request cogsys/dare2del/demonstrator!25

for endpoints expecting none

for endpoints expecting payload

Apply the new system tests on the changed API

See merge request cogsys/dare2del/demonstrator!24

add error response json schema

add response schema and link in documentation

simplifies validation of schemas with "$ref"

add response schema and link in documentation

update api tests

GitLab identifies tests only by the Postman
test name. The reqest name is ignored. Tests
 with duplicate names are discarded! Thus
tests with the same name in different
Requests or in different data iterations are
ignored. Thus, test names now include the
Request name and the iteration.