polynomial type with non Eigen types

[stonneau]

In the current implementation of the polynomial class, the coefficients are stored using an Eigen matrix and not a vector. If other types are used (such as linear_variable), the polynomial class will not work. We should either change this or remove the point type form the template definition as it is useless. The coefficients should be stored as std::vector.

Is there any reasons this has been done differently ? @JasonChmn @proyan