Fixing unstable nonstationary norm, adding utils.visual, and doing some code refactoring

[WenjieDu]

What does this PR do?

1. add pypots.nn package and pypots.nn.functional;
2. rename pypots.modules into pypots.nn.modules;
3. add pypots.utils.visual package;
4. fix Current non-stationary normalization may result in NaN values #265;

Before submitting

• This PR is made to fix a typo or improve the docs (you can dismiss the other checks if this is the case).
• Was this discussed/approved via a GitHub issue? Please add a link to it if that's the case.
• I have commented my code, particularly in hard-to-understand areas.
• I have written necessary tests and already run them locally.

[coveralls]

Pull Request Test Coverage Report for Build 7178090015

• 24 of 179 (13.41%) changed or added relevant lines in 14 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-2.1%) to 80.875%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/imputation/timesnet/modules/core.py	5	7	71.43%
pypots/nn/functional/normalization.py	5	30	16.67%
pypots/utils/visual.py	0	54	0.0%
pypots/utils/visualization.py	0	74	0.0%

Totals
Change from base Build 7157280175:	-2.1%
Covered Lines:	4808
Relevant Lines:	5945

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💛 - Coveralls