#96 Dynamic diagrams supports Index(), LastIndex() and SetIndex()

[kirchsth]

Instead of the increment() and setIndex() functions the user can use Index(), LastIndex() and SetIndex() directly in the RelIndex(...) macro. (The already existing Increment() and setIndex() functions works unchanged)

The new notation is shorter (and I think simpler) like below

Rel_D(customer, app, "Updates his profile using", "HTTPS")
Rel(app, customer_service, "Updates customer information using", "JSON/HTTPS")
Rel_R(customer_service, customer_db, "Stores data in", "JDBC")

RelIndex_D(Index()-1, customer_service, message_bus, "Sends customer update events to", "async")
RelIndex_U(LastIndex()-2, customer_service, app, "Confirm update to", "async")

RelIndex_L(Index()-1, message_bus, reporting_service, "Sends customer update events to", "async")
RelIndex(Index()-1, reporting_service, reporting_db, "Stores data in")

RelIndex_R(SetIndex(5)-2, message_bus, audit_service, "Sends customer update events to", "async")
RelIndex(Index()-2, audit_service, audit_store, "Stores events in")

compared to the existing one.

Rel_D(customer, app, "Updates his profile using", "HTTPS")
Rel(app, customer_service, "Updates customer information using", "JSON/HTTPS")
Rel_R(customer_service, customer_db, "Stores data in", "JDBC")

RelIndex_D($index-1, customer_service, message_bus, "Sends customer update events to", "async")
RelIndex_U($index-2, customer_service, app, "Confirm update to", "async")
increment()

RelIndex_L($index-1, message_bus, reporting_service, "Sends customer update events to", "async")
increment()
RelIndex($index-1, reporting_service, reporting_db, "Stores data in")

setIndex(5)
RelIndex_R($index-2, message_bus, audit_service, "Sends customer update events to", "async")
increment()
RelIndex($index-2, audit_service, audit_store, "Stores events in")

This PR need the new PlantUML verson V1.2020.24 (it can be already tested with the online server http://www.plantuml.com/plantuml/ it has the V1.2020.24beta4)