Anopheles refactor PCA functions

[alimanfoo]

• Continue working towards #366 here tackling the PCA computation and plotting functions.
• Refactor other plotting functions to use common symbol and color setup code where possible.
• Add notebook to check plotting of diversity stats.

[codecov[bot]]

Codecov Report

Attention: 5 lines in your changes are missing coverage. Please review.

Comparison is base (aa35798) 98.09% compared to head (b646b3b) 98.01%.

:exclamation: Current head b646b3b differs from pull request most recent head ed7b7fc. Consider uploading reports for the commit ed7b7fc to get more accurate results

Files	Patch %	Lines
malariagen_data/anoph/sample_metadata.py	92.85%	5 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #481 +/- ## ========================================== - Coverage 98.09% 98.01% -0.08% ========================================== Files 28 29 +1 Lines 2200 2369 +169 ========================================== + Hits 2158 2322 +164 - Misses 42 47 +5 ```

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