ENH: Add repr for inverse

This PR might betray my knowledge of forward/inverse stuff. Do we need anything else in the `repr`?
This will not work well with @leggitta's SourceSpaces changes. @leggitta can you have a look and talk with @mainakjas about how to modularize the `SourceSpaces` `__repr__` code so that @mainakjas can just call to your function? This and your functions should each probably call a function like `_get_name_str` or so that sets up a name string for the `Volume with %s points` part.