model: Fix incompliant inheritance of OperationVariable from SubmodelElement

This includes reverting fc9f4ae6 and 1ba9dff0 and some modifications
to JSON serialization.
1 job for fix/operation_variable_bases in 45 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #588862
test

00:00:45

90.0%