experimental: add data collection only decorator supporting mpi

[guanyilun]

For example,

import numpy as np
from galilei.sampling import build_samples
from galilei.experimental import collect

@collect(samples=build_samples(
    {'a': [0, 2], 'b': [0, 2]}, 100000
), save="test.pkl", mpi=True)
def test_em(a, b):
    x = np.linspace(0, 10, 100)
    return np.sin(a*x) + np.sin(b*x)

and suppose this script is called test.py. One could run it with

mpirun -n 10 python test.py

to perform parallel collection. Note that this requires mpi4py which will not be installed by default.

[codecov-commenter]

Codecov Report

Patch coverage has no change and project coverage change: -1.02 :warning:

Comparison is base (0308bfb) 12.26% compared to head (2e7baa6) 11.24%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #13 +/- ## ========================================== - Coverage 12.26% 11.24% -1.02% ========================================== Files 8 9 +1 Lines 375 409 +34 ========================================== Hits 46 46 - Misses 329 363 +34 ``` | [Impacted Files](https://codecov.io/gh/guanyilun/galilei/pull/13?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yilun+Guan) | Coverage Δ | | |---|---|---| | [galilei/experimental.py](https://codecov.io/gh/guanyilun/galilei/pull/13?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yilun+Guan#diff-Z2FsaWxlaS9leHBlcmltZW50YWwucHk=) | `0.00% <0.00%> (ø)` | | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yilun+Guan). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Yilun+Guan)

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