Fix and test the integral propery with np_vstack

[MarcT0K]

This PR adds unit tests to check the consistency of the integral property for various operations on secure fixed-point numbers. The current tests covers the basic arithmetic operations (additions and multiplications) and the array stacking operations (np_vstack, np_hstack, np_stack, np_column_stack).

It also fixes the inconsistency in np_vstack.

See #67 for more details.

[codecov-commenter]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.07% :tada:

Comparison is base (1636d55) 92.88% compared to head (8e61824) 92.95%.

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Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #69 +/- ## ========================================== + Coverage 92.88% 92.95% +0.07% ========================================== Files 16 16 Lines 8869 8874 +5 ========================================== + Hits 8238 8249 +11 + Misses 631 625 -6 ``` | [Files Changed](https://app.codecov.io/gh/lschoe/mpyc/pull/69?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Berry+Schoenmakers) | Coverage Δ | | |---|---|---| | [mpyc/runtime.py](https://app.codecov.io/gh/lschoe/mpyc/pull/69?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=Berry+Schoenmakers#diff-bXB5Yy9ydW50aW1lLnB5) | `91.53% <100.00%> (+0.21%)` | :arrow_up: |

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[MarcT0K]

Also, I've shortened the syntax for the tests to make it more like the other tests.

Looking at your commit, I've just discovered that there was some sort of aliases between np.* and mpc.np_*. I hadn't seen the trick in sectypes.py to overload the functions for secure arrays. This will simplify my own codes :sweat_smile:

I'll now prepare a few other PRs to cover all numpy operations.

[lschoe]

OK! And yes the use of np.* calls is the intended way and can be used in most cases. Check out the demos listed in np-run-all and other unit tests to see all kinds of examples. Only in a few cases the use of mpc.np_* calls is needed.