Support for pandas DataFrame

[lepmik]

To extend our support for data structures and variable length strings, storage with feather which support saving pandas DataFrames has been implemented.

• Storing with feather is invoked when a DataFrame is presented to the API
• All previous functionality still works where everything that is not a DataFrame invokes regular behavior with numpy memmaps.

[codecov[bot]]

Codecov Report

Merging #89 into dev will increase coverage by 0.48%. The diff coverage is 98.65%.

@@            Coverage Diff             @@
##              dev      #89      +/-   ##
==========================================
+ Coverage   95.57%   96.05%   +0.48%     
==========================================
  Files          11       13       +2     
  Lines        1604     1902     +298     
==========================================
+ Hits         1533     1827     +294     
- Misses         71       75       +4

Impacted Files	Coverage Δ
tests/test_dataset.py	100% <100%> (ø)	:arrow_up:
tests/test_links.py	97.95% <97.95%> (ø)
tests/test_dataframe.py	98.96% <98.96%> (ø)

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