Replace the hard coded tau indices by a central way of constructing it.

[mlaveaux]

The LTS trait allows one to check whether a given label is the tau label, which internally is always index 0 and there could be multiple labels, but there should also be a way to construct the tau action.

[mlaveaux]

This mostly concerns the FeaturedLabel, which assigns a BDD to each LTS label as well, as constructing it requires access to the BDDManager. Currently, this cannot be done and as such it simply panics.

Related to this discussion it might also be good to remove the hidden label option (--tau=ACTIONS) completely. This option is generally confusing as the counter examples returned from merc-lts only apply to the inputs with these hidden actions applied. As such, this option must then be used consistently in all places where LTSs are being handled. However, currently this option is also used to deal with the differences between the i and tau in the AUT files generated by CADP and mCRL2. For this we could consider introducing two different AUT formats (the AUT and mCRL2 AUT formats) to be provided in --format.

[mlaveaux]

I removed the --tau option, and introduced an aut-mcrl2 format. It could be reintroduced in specific cases where it makes sense such that checking for weak trace inclusion, where the weak trace could be mapped back to a concrete trace.