remove getstate and setstate from Stack, and base argument from init

[bmaranville]

There was a TODO in the Stack class, indicating that the custom __getstate__ and __setstate__ methods might not be needed.

These methods were making it difficult to subclass Stack without implementing even more specialized versions of the methods.

After removing them, deepcopy and serialize/deserialize seem to work fine.
Here is a test script to run and check. It shows that

• the thickness Calculation function is properly linked in the Stack.thickness.slot after deepcopy and serialize/deserialize
• the thickness attribute Parameter remains properly linked in Expression objects after deepcopy and serialize/deserialize

from refl1d.names import Slab, Stack, SLD
from copy import deepcopy
from bumps.serialize import serialize, deserialize
import pickle
import dill

sample = Slab(SLD(rho=10), thickness=10) | Slab(SLD(rho=10), thickness=20) | Slab(SLD(rho=10), thickness=30)
thickness_plus = sample.thickness + 100 # expression

ser_t, ser_s = deserialize(serialize([thickness_plus, sample]))
assert(ser_t.value == 160)
ser_s[0].thickness.value += 40
assert(ser_t.value == 200)

dc_t, dc_s = deepcopy([thickness_plus, sample])
assert(dc_t.value == 160)
dc_s[0].thickness.value += 40
assert(dc_t.value == 200)

pickle_t, pickle_s = pickle.loads(pickle.dumps([thickness_plus, sample]))
assert (pickle_t.value == 160)
pickle_s[0].thickness.value += 40
assert (pickle_t.value == 200)

dill_t, dill_s = dill.loads(dill.dumps([thickness_plus, sample]))
assert (dill_t.value == 160)
dill_s[0].thickness.value += 40
assert (dill_t.value == 200)

assert(thickness_plus.value == 160)
sample[0].thickness.value += 40
assert(thickness_plus.value == 200)

[hoogerheide]

Looks good. Serialization/deserialization on a deepcopy of a regular model is functional.