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