It is allowed to build CQM object with quadratic interaction between real variables without any error raised

[alexzucca90]

Quadratic interactions between continuous variables are not supported.
However, it is possible to build a CQM object with quadratic interaction between two real variables by "bypassing" some checks. This happens if we build a model from iterable

To reproduce

import dimod

cqm = dimod.CQM()

cqm.add_variable('REAL', 'x')
cqm.add_variable('REAL', 'y')
cqm.add_constraint([('x', 'y', 1)], ">=", -1, label="c")
print(cqm.constraints['c'].lhs)

We expect to have an error like ValueError: REAL variables (e.g. 'x') cannot have interactions.