Change class name: PolynomialNarmax to FROLS

[wilsonrljr]

Currently, the PolynomialNarmax uses the FROLS algorithm to select the structure of the model. The new MetaMSS and AOLS algorithms are two alternatives to FROLS algorithm and also build polynomial models.

SysIdentPy focuses on model structure selection algorithms, so the FROLS name is more intuitive, especially now with new structure selection methods being added.

Moreover, NARMAX models can be built with different basis functions and polynomial is only one of the many available. Renaming the class will help even when new base functions are added in SysIdentPy.