Nested saved persistent hashes

See merge request !3

UniqueNameGenerator: Add a _name_added() callback.

See merge request !2

Prep for Flake8 during CI

See merge request !1

[bugfix] Use base classes for ImmutableRecord in correct order

A very weird bug: The order of base classes actually matters here,
because if __hash__ and __eq__ are defined in the wrong order,
__hash__ will get deleted (in 3). MWE:

```
class X(object):
    def __eq__(self, other):
        return True

class Y(object):
    def __hash__(self):
        return 0

class Z1(Y,X):
    pass

class Z2(X,Y):
    pass

hash(Z1())
hash(Z2())
```

Import numpy or skip for test_spatial_btree

test_spatial_btree currently fails if numpy is not available.

test_p_convergence_verifier uses the pytest importorskip function
to skip the test when numpy is not available.

Do the same for test_spatial_btree.