Create a mask from regions

I'm not finding in the documentation any function to create a mask from a set of regions. Seems like we should have that, and maybe the inverse as well.

Something like this...

```
def mask_from_regions(spectrum,regions):
mask = np.zeros(spectrum.spectral_axis.shape,dtype=np.bool)
for r in regions:
submask = (spectrum.spectral_axis>r.lower) & (spectrum.spectral_axis<=r.upper)
mask = mask | submask
spectrum.mask = mask
```

Start by reviewing the documentation and existing spectrum masking APIs for a suitable entry point for mask_from_regions. Compare the requested region-bound behavior with current mask handling, and clarify whether the inverse operation is required. Done means the supported region-to-mask behavior is documented and covered by tests.

Written by the indexing model from the issue text.