Implement soft link and external link

[lepmik]

• Link classes live in links.py module
• They are subclass of the abstract class Link
• Functionality should match h5py

Should fix https://github.com/CINPLA/exdir/issues/1 Aims for compatibility with NWB https://github.com/NeurodataWithoutBorders/pynwb/issues/629

[codecov[bot]]

Codecov Report

Merging #87 into dev will increase coverage by 0.13%. The diff coverage is 97.95%.

@@            Coverage Diff             @@
##              dev      #87      +/-   ##
==========================================
+ Coverage   95.57%   95.71%   +0.13%     
==========================================
  Files          11       12       +1     
  Lines        1604     1702      +98     
==========================================
+ Hits         1533     1629      +96     
- Misses         71       73       +2

Impacted Files	Coverage Δ
tests/test_links.py	97.95% <97.95%> (ø)

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