atlas_io is an adaptor library when eckit_codec is available

[wdeconinck]

When eckit is compiled with feature ECKIT_CODEC enabled, then atlas_io will with this PR now delegate all calls to eckit_codec.

This ensures a smooth transition for downstream projects of atlas_io (such as atlas-orca, multio) to migrate to use eckit_codec directly.

This PR supersedes PR #180

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 80.07%. Comparing base (5dc85c9) to head (d244194).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #181 +/- ## ======================================== Coverage 80.07% 80.07% ======================================== Files 859 859 Lines 63682 63682 ======================================== Hits 50993 50993 Misses 12689 12689 ```

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