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

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`.

Start by running the reproduction in the issue and tracing the CQM.add_constraint path that accepts the iterable interaction. Check where REAL-variable interaction validation is performed; done means the example raises the expected ValueError instead of building the constraint.

Written by the indexing model from the issue text.