Support OpenFF's constraints

[martimunicoy]

Description

Currently, we are ignoring the parameters typed as Constraints in label_molecules()' parameters. We realized that these constraints are used in some force fields as in the tip3p parameters for water.

Solution

Constraints in PELE are not defined in the Impact template file. They are defined in the PELE control file. So, a solution could defining the constraints through bonds with huge force constants.