Make hypervolume a function hvc.py

Currently using hypervolume looks like:

import hypervolume as hv
hypervolume = hv.Hypervolume()
result = hypervolume(data)

which is counterintuitive. I prefer it simpler:

import hypervolume as hv
result = hv.hypervolume(data)

Resolution options:

- Move library loading code (`__init__` method) into the function
- Load the library on module level (once per import)
- Leave the class but do the hypevolume=Hypervolume call on module scope