Added initial Matplotlib-based GUI slicer

@ChrisBeaumont @keflavich - try this out! You just need to do:

```
from pvextractor.gui import PVSlicer
pv = PVSlicer('L1448_13CO.fits')
pv.show()
```

To draw a slice, click once to define the first point, a second time to define the end point, and a third time to define the width.

If you have ``spectral_cube`` installed, it will use that for the I/O. Still lots to do, including FITS export, but at least this is a proof of concept.

Ideas welcome!