Replace numpy.matrix with numpy.ndarray

[dnerini]

closes #406

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 83.67%. Comparing base (d77fe73) to head (0919ba2).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #407 +/- ## ======================================= Coverage 83.67% 83.67% ======================================= Files 159 159 Lines 12649 12653 +4 ======================================= + Hits 10584 10588 +4 Misses 2065 2065 ``` | [Flag](https://app.codecov.io/gh/pySTEPS/pysteps/pull/407/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pySTEPS) | Coverage Δ | | |---|---|---| | [unit_tests](https://app.codecov.io/gh/pySTEPS/pysteps/pull/407/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pySTEPS) | `83.67% <100.00%> (+<0.01%)` | :arrow_up: | 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=pySTEPS#carryforward-flags-in-the-pull-request-comment) to find out more.

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