Feature/active learning pce

Active learning for polynomial chaos expansion New active learning class for PCE

Description

Added a new class for adaptive sequential sampling based on Theta criterion for PCE. Two additional functions in PCE class for standardization of a sample and corresponding PDF.

Types of changes

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

SURGroup / UQpy