Rename save_model() and load_model() into save() and load()

[WenjieDu]

What does this PR do?

Replace save_model() and load_model() with save() and load(). Old functions save_model() and load_model() are still kept for compatibility and will be removed in the future.

Before submitting

• 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 7028965482

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A quick fix for this PR: rebase it; your next report should be accurate.

• 21 of 26 (80.77%) changed or added relevant lines in 1 file are covered.
• 4 unchanged lines in 4 files lost coverage.
• Overall coverage decreased (-0.1%) to 83.343%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pypots/base.py	21	26	80.77%

Files with Coverage Reduction	New Missed Lines	%
pypots/base.py	1	69.68%
pypots/classification/base.py	1	74.07%
pypots/cli/env.py	1	65.31%
pypots/data/base.py	1	80.77%

Totals
Change from base Build 7028744993:	-0.1%
Covered Lines:	4443
Relevant Lines:	5331

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💛 - Coveralls