Handle same compounds in products or reactants in padmet_to_padmet.

[ArnaudBelcour]

Some reactions like RXN-2103 contain the same compounds multiple times as products or reactants:

1-4-alpha-D-galacturonosyl + WATER -> 1-4-alpha-D-galacturonosyl + 1-4-alpha-D-galacturonosyl

But by using padmet_to_padmet, this will be converted into:

1-4-alpha-D-galacturonosyl + WATER -> 1-4-alpha-D-galacturonosyl

This is due to the use of the _addRelation function of 'padmet/classes/padmetSpec.py' which does not allow to have the same relations multiple times.

There should be a way to add the same multiple relations of 'consumes' or 'produces' between a reaction and its compounds without adding all the other relations multiple times.

Issue found in 6767816.