Add datareader.size

[ChrisspyB]

Depends on PR: ecmwf/fdb#50

Add size to datareader. This allows nice things like:

    dh = fdb1.retrieve(request)
    blob = dh.read() # no argument now reads whole handle.
    fdb2.archive(blob)
    fdb2.flush()

[FussyDuck]

All committers have signed the CLA.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.77%. Comparing base (0d6c2a4) to head (48af9a8).

Additional details and impacted files

@@           Coverage Diff            @@
##           develop      #37   +/-   ##
========================================
  Coverage    94.77%   94.77%           
========================================
  Files            3        3           
  Lines          134      134           
========================================
  Hits           127      127           
  Misses           7        7           

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