default names for output files are inconsistent

For a module called `swigexample`
```
swig -matlab example.i
```
generates `example_wrap.cxx`, a folder `+swigexample`, and `m` files calling `swigexampleMEX`.

This means that either you have to use `mex example_wrap.cxx -output swigexampleMEX`, or use `swig -mexname example_wrap` or use `swig -o swigexampleMEX.cxx`