Problems with Trend Best Fit

[oleibman]

There are at least 2 problems. The first is minor, using the wrong property in a single statement; this is addressed by @quonly in PR #4339.

A more serious problem is that PolynomialBestFit just doesn't work. Phpstan and Scrutinizer both point out, correctly, several places where a property typed as array is attempted to be used as float. I am not sure how to resolve these problems. For now, I am disabling the use of Polynomial Best Fit by throwing an exception if it is attempted.