pins2lts-mc / treedbs with state vector of size 1

[jacopol]

For small examples,*2lts-mc complains that a state vector of length 1 cannot be handled:
opaal2lts-mc, file treedbs-ll.c, line 488: assertion "nNodes >= 2" failed: Tree vectors too small: 1

This could be improved by adding a dummy state slot dummy:{1}.

There are several use cases for this:

• users may try out the tool with a small example
• we want to run unit tests on models from external language providers
• we should support model checking of pre-computed (explicit) state spaces

[jacopol]

A workaround that seems to work is currently: *2lts-mc --state=table.
So an alternative quick solution could be to change the default state representation when the state vector has length 1.