Fix deserialization by applying RoiWrapper

[JonasHell]

Should fix the issue by applying the RoiWrapper while unpickling.
Sample Code:

import numpy as np
import pickle
from torch_em.data import RawDataset, SegmentationDataset

path = '/g/kreshuk/hellgoth/domain_adaptation/data/cremi/sample_A_20160501_copy.hdf5'
raw_key = 'volumes/raw' # (125, 1250, 1250)
label_key = 'volumes/labels/neuron_ids' # (125, 1250, 1250)

# RawDataset
ds_raw = RawDataset(
    raw_path=path,
    raw_key=raw_key,
    patch_shape=(32, 256, 256),
    roi=np.s_[:50, :1000, :1000]
)

print(ds_raw)
print(ds_raw.raw.shape) # (50, 1000, 1000)

with open('test_file', 'wb') as f:
    pickle.dump(ds_raw, f)

with open('test_file', 'rb') as f:
    ds_loaded_raw = pickle.load(f)

print(ds_loaded_raw)
print(ds_loaded_raw.raw.shape) # old behavior: (125, 1250, 1250), new behavior: (50, 1000, 1000)


# SegmentationDataset
ds_seg = SegmentationDataset(
    raw_path=path,
    raw_key=raw_key,
    label_path=path,
    label_key=label_key,
    patch_shape=(32, 256, 256),
    roi=np.s_[:50, :1000, :1000]
)

print(ds_seg)
print(ds_seg.raw.shape) # (50, 1000, 1000)
print(ds_seg.labels.shape) # (50, 1000, 1000)

with open('test_file2', 'wb') as f:
    pickle.dump(ds_seg, f)

with open('test_file2', 'rb') as f:
    ds_loaded_seg = pickle.load(f)

print(ds_loaded_seg)
print(ds_loaded_seg.raw.shape) # old behavior: (125, 1250, 1250), new behavior: (50, 1000, 1000)
print(ds_loaded_seg.labels.shape) # old behavior: (125, 1250, 1250), new behavior: (50, 1000, 1000)

[constantinpape]

Thanks for working on this, but already fixed in #55.