fix(ProcessInstanceAssert): make sure calls to calledProcessInstance do not affect last asserted process instance from pov of test class

Affects #28
test(ProcessEngineTests): test calledProcessInstance

Affects #28
docs(ProcessEngineTests): new helper methods to deal with called process instances

Closes #28
chore(pom): switch default version of camunda-bpm to 7.1.0-Final

Closes #24