Tests for Distributions module

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I created tests for the Distributions module and corrected mistakes in several places in the Distributions module (certain functions for certain specific distributions that were not tested in the example notebook were creating errors).

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How Has This Been Tested?

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Types of changes

What types of changes does your code introduce? Put an x in all the boxes that apply:

[x ] Bug fix (non-breaking change which fixes an issue)
[x ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

Go over all the following points, and put an x in all the boxes that apply. If you're unsure about any of these, don't hesitate to ask. We're here to help!

[x ] My code follows the code style of this project.
[ ] My change requires a change to the documentation.
[x ] I have updated the documentation accordingly.
[x ] I have read the CONTRIBUTING document.
[x ] I have added tests to cover my changes.
[x ] All new and existing tests passed.

SURGroup / UQpy

Feature/tests distributions #90