The plugins used to keep a reference to a config object as provided by IOManager. But this was triggering some warnings (reference to an object that might disappear outside of the constructor), therefore it has been replaced by a const copy (it is not yet 100% sure if this is really better, but at least the compilers won't complain).

When a user was provinding IMISIO with a non-existing station name, the error messages were quite cryptic. In order to properly tell the user what happened, the code has been slightly reorganized. Some code clean up also took place in IMISIO.