Add PyramidKVPress

[figuremout]

PR description

Add support for PyramidKVPress (#65) based on the original author's code. It maintains pyramid-like cache sizes, allocating more cache budget to lower layers and less to higher layers.

It performs slightly worse on the RULER 4k dataset than SnapKV.

Checklist

• Tests are working (make test)

=============== 100 passed, 23 skipped, 2 warnings in 59.27s ================

• Code is formatted correctly (make style, on errors try fix with make format)

Found 27 errors in 2 files (checked 42 source files)
make: *** [Makefile:28: style] Error 1

But none of them related to my PR.

• Copyright header is included
• All commits are signed-off using git commit -s
• (new press) mypress_press.py is in the presses directory
• (new press) MyPress is in __init__.py
• (new press) README.md is updated with a 1 liner about the new press in the Available presses section
• (new press) new press is in the default_presses list in tests/default_presses.py

[maxjeblick]

Thanks a lot for your PR for PyramidKVPress! I left a few comments in the code.
Feel free to comment with any follow-up questions/remarks.

[maxjeblick]

LGTM, thanks a lot for adding PyramidKVPress!