Examples: Add example on process outputs

[sphuber]

The example launches an ArithmeticAddCalculation, polls for its status until it is terminated using the /processes end-point and then retrieves its outputs using /processes/outputs.

[sphuber]

Blocked by #54

[codecov-commenter]

Codecov Report

Base: 93.32% // Head: 93.16% // Decreases project coverage by -0.15% :warning:

Coverage data is based on head (47ddeb4) compared to base (51906f6). Patch coverage: 83.33% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #55 +/- ## ========================================== - Coverage 93.32% 93.16% -0.16% ========================================== Files 23 23 Lines 929 937 +8 ========================================== + Hits 867 873 +6 - Misses 62 64 +2 ``` | Flag | Coverage Δ | | |---|---|---| | pytests | `93.16% <83.33%> (-0.16%)` | :arrow_down: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidateam#carryforward-flags-in-the-pull-request-comment) to find out more. | [Impacted Files](https://codecov.io/gh/aiidateam/aiida-restapi/pull/55?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidateam) | Coverage Δ | | |---|---|---| | [aiida\_restapi/routers/process.py](https://codecov.io/gh/aiidateam/aiida-restapi/pull/55/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=aiidateam#diff-YWlpZGFfcmVzdGFwaS9yb3V0ZXJzL3Byb2Nlc3MucHk=) | `96.55% <83.33%> (-3.45%)` | :arrow_down: | 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=aiidateam). 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=aiidateam)

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[sphuber]

Superseded by #57