rename: AbstractSysDyn.ofts -> .ts

- easier to use and read
- prepares for allowing AbstractSysDyn to be either an FTS or OpenFTS (or possibly some other type of labeled graph in the future)
- allows for shorter name of forthcoming: AbstractSysDyn.ppp2ts
new: AbstractSysDyn.ppp2ts

map each Region index of list AbstractSysDyn.ppp.list_region to its corresponding state in AbstractSysDyn.ts
(abstract.discretization.discretize creates str states)

useful for explicitly storing the map
avoid mutable default argument disc_params

because if AbstractSysDyn is initialized w/o it twice, then the disc_params dict object will be common for the two discretizations.
Any updates to the dict (w/o setting it equal to a completely new dict) will result in mixing the options between the 2 different abstractions.