Add interactions iterator

[cbouy]

Adds custom .interactions() iterator for values in fp.ifp that allows doing:

for index, ifp in fp.ifp.items():
    for interaction in ifp.interactions():
        print(interaction.ligand, interaction.protein, interaction.name, interaction.metadata)

[codecov[bot]]

Codecov Report

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

Project coverage is 94.47%. Comparing base (9be3cea) to head (cc30735).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #210 +/- ## ========================================== + Coverage 94.43% 94.47% +0.04% ========================================== Files 20 20 Lines 1473 1484 +11 ========================================== + Hits 1391 1402 +11 Misses 82 82 ```

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