Adding moving_std to computations

[Asanto32]

• resolves #28

Implement the moving standard deviation function in computations.py. Code for the function and tests are based on the moving_mean function. Minor fixes to test_computations: using global vars for SIGNAL_LENGTH and EPOCH_LENGTH

[codecov[bot]]

Codecov Report

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

Project coverage is 100.00%. Comparing base (2c05605) to head (439c97a).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## development #34 +/- ## ============================================= Coverage 100.00% 100.00% ============================================= Files 4 4 Lines 120 132 +12 ============================================= + Hits 120 132 +12 ```

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