Uniform potentials are not converted into tables

[Francisco Javier Díez]

When evaluating this ID, I get an error message: “Numeric variable U makes it impossible to project this uniform potential into a table.” At the moment, the problem could be solved by converting that potential into a TablePotential of no variables, i.e., a constant, with value 0.

In the future, UniformPotential should be renamed as DummyPotential.

[Manuel Arias]

The uniform potentials, when there is a continuous conditioned variable, remove the continuous variable when projecting, avoiding throwing an exception.