new arg statevar to synth.states2ints

statevar specifies the desired name for the int-valued variable representing the transition system state.

also some minor refactoring
bugfix: use state_ids, not states
convert environment transition system to GR(1)

synth.synthesize accepts two transition systems: env and sys.
The initial states in each of env, sys can be ignored using synthesize args:     ignore_env_init, ignore_sys_init.

The environment state variable is called 'eloc' to avoid conflicts with system state naming, in case both sys and env are provided to synthesize.

Note that selecting bool_states requires that the user named states of env and sys transition systems differently.
Otherwise variable name conflicts will result between sys bool states and env bool states.

The main differences with how a sys TS is treated are:
    - an env dead-end state can result in a trivially realizable spec, so a warning is printed in this case
    - the system might be able to block the environment, rendering False the assumption, hence trivially realizable the spec. Requiring that always exactly one sys action be True in the guarantee need not be enforced in this case. The implementation escapes this case by deconstraining env to transition freely in case sys violates the exactly one requirement on sys_actions.
update switching examples to use env, sys transition systems

- developer example is unrealizable

- robot_controlled_switching has nondeterminism which is fully controlled by the system. This case arises when the actual system will use different actions, but these are lumped into the same action as an abstraction to reduce the number of problem variables. The correct action needed can still be recovered by the start- and end- states of each transition.

- only_switching_controlled models the case when the system controls only the switching mode. So if from a state and for a selected system action more than one next states are possible, the environment chooses which one will be next. This is modeled by using an env TS.