Loading cache for resampling scene fails with numpy 1.16.3

[observingClouds]

Describe the bug
With the new release of Numpy (1.16.3) the

unpickling object arrays
when loading *.npy or *.npz files now requires an explicit opt-in.
This backwards incompatible change was made in response to
CVE-2019-6446 https://nvd.nist.gov/vuln/detail/CVE-2019-6446_.

(https://github.com/numpy/numpy/releases)

Due to this change the function call scene.resample() fails when cache_dir is given and used to load an existing cache.

To Reproduce

from satpy import Scene

datafile = SomeGOES16File.nc
scn = Scene(reader="abi_l1b", filenames=[datafile])
scn.load(['C13'])
new_scn = scn.resample(scn.min_area(), cache_dir='./') #Writing cache

new_scn2 = scn.resample(scn.min_area(), cache_dir='./') #Using cache

Expected behavior
new_scn should include resampled scene when using cache_dir and resample cache exists

Actual results

new_scn = scn.resample(scn.min_area(), cache_dir='./')
  File "../python3.6/site-packages/satpy/scene.py", line 1101, in resample
    reduce_data=reduce_data, **resample_kwargs)
  File "../python3.6/site-packages/satpy/scene.py", line 1056, in _resampled_scene
    res = resample_dataset(dataset, destination_area, **kwargs)
  File "../python3.6/site-packages/satpy/resample.py", line 916, in resample_dataset
    new_data = resample(source_area, dataset, destination_area, fill_value=fill_value, **kwargs)
  File "../python3.6/site-packages/satpy/resample.py", line 879, in resample
    res = resampler_instance.resample(data, **kwargs)
  File "../python3.6/site-packages/satpy/resample.py", line 271, in resample
    cache_id = self.precompute(cache_dir=cache_dir, **kwargs)
  File "../python3.6/site-packages/satpy/resample.py", line 352, in precompute
    self.load_neighbour_info(cache_dir, mask=mask, **kwargs)
  File "../python3.6/site-packages/satpy/resample.py", line 383, in load_neighbour_info
    new_cache = dict(cache.items())
  File "../python3.6/_collections_abc.py", line 744, in __iter__
    yield (key, self._mapping[key])
  File "../python3.6/site-packages/numpy/lib/npyio.py", line 262, in __getitem__
    pickle_kwargs=self.pickle_kwargs)
  File "../python3.6/site-packages/numpy/lib/format.py", line 692, in read_array
    raise ValueError("Object arrays cannot be loaded when "
ValueError: Object arrays cannot be loaded when allow_pickle=False

Environment Info:

• OS: Linux
• Satpy Version: 0.15.1
• PyResample Version: 1.12.3

[observingClouds]

Possible solution
Change the following line 381 in satpy/resample.py from
cache = np.load(filename, mmap_mode='r')
to
cache = np.load(filename, mmap_mode='r', allow_pickle=True)

[pnuu]

Hmm. Maybe we should finally have a look at using zarr for the caches. @djhoese what do you think?

[djhoese]

@pnuu Yes, but we should include @observingClouds fix for the next bug release too. I've asked @observingClouds to resubmit their PR to this repository (currently only on their own fork).