Add ecfs source

[b8raoult]

Works with

ekd.from_source('ecfs', '/path/to/data')

and

ekd.from_source('url', 'ec:/path/to/data')

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 90.14%. Comparing base (87232b6) to head (6751922).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #568   +/-   ##
========================================
  Coverage    90.14%   90.14%           
========================================
  Files          149      149           
  Lines        10972    10972           
  Branches       519      519           
========================================
  Hits          9891     9891           
  Misses         914      914           
  Partials       167      167           

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[sandorkertesz]

This is a bit dangerous, since other sources like ecfs do not necessarily support "url" source features like parts or streams.

[b8raoult]

You can remove that if you wish

[sandorkertesz]

Why is it needed here?