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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.47%. Comparing base (9be3cea) to head (cc30735).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            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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