prism-converter: Add option for model vars as initial partition

This adds two options to `prism-converter`: `--states-file` allows to
specify a `.sta` file as exported from PRISM and
`--partition-on-variables` allows to specify a list of variables from
the model that should be used to determine the initial partition.

This commit only adds the parsing and interpretation steps, the
resulting partition is not yet reflected in the output.