[Child] covmat to docplex binary variables

[gcattan]

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[gcattan]

@toncho11
this is a good follow-up based on your previous PR.
First step, would probably be to create a docplex.py file inside the utils folder.
Inside this file you can report here the method you created in docplex_test.py to create continuous variable matrices, except it should accept a docplex problem as a parameter.

def covmat_var(prob, channels, name='cont_covmat'):
        ContinuousVarType.one_letter_symbol = lambda _: 'C'
        return prob.continuous_var_matrix(keys1=channels, keys2=channels,
                                          name=name, lb=-prob.infinity)

We can then think about how to create a similar method for integer and binary variables.