add support for multiple-value (array) outputs

[fmigneault]

fixes #25

todo

• [x] fix tests (as applicable)
• [x] remove remaining #25 comments
• [x] update doc about multiple-outputs (https://github.com/crim-ca/weaver/blob/325aa4fdb3e89457c76fb8a05c7a602db8f5bf7d/docs/source/processes.rst#multiple-outputs)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 84.07643% with 25 lines in your changes missing coverage. Please review.

Project coverage is 85.78%. Comparing base (3e9a8ae) to head (73b7aed).

Files	Patch %	Lines
weaver/wps_restapi/jobs/utils.py	71.42%	8 Missing and 8 partials :warning:
weaver/processes/convert.py	87.87%	1 Missing and 3 partials :warning:
weaver/wps_restapi/colander_extras.py	88.88%	2 Missing and 1 partial :warning:
weaver/processes/wps_package.py	93.54%	1 Missing and 1 partial :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #662 +/- ## ========================================== + Coverage 85.74% 85.78% +0.03% ========================================== Files 79 79 Lines 18862 18944 +82 Branches 2912 2928 +16 ========================================== + Hits 16174 16251 +77 - Misses 1931 1936 +5 Partials 757 757 ```

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