Be more paranoid about initializing the RNG

This is an attempt to enforce reproducible runs. Whenever from_random is called without a preceding random.init(), an exception will be raised.

@parras, does this look OK in principle?

I also realized that there are still some direct calls to np.random functionality within nifty6, which I think is bad. We should replace this soon.

nifty6/__init__.py

 from .version import __version__
 
+from . import random
+random.seed(42)
+
 from .domains.domain import Domain
 from .domains.structured_domain import StructuredDomain
 from .domains.unstructured_domain import UnstructuredDomain