parameter system: some implementation related improvments

- don't crash the simulation if there was a syntax error when
  specifying the parameter.
- enforce that the types passed to REGISTER_PARAM and GET_PARAM are
  identical.
- parse and retrieve all parameters at the time of registration. This
  means that there is no runtime penalty of the first call to GET_PARAM
  compared to the successive ones anymore.

the latter two items are just welcome side effects of the first one!