KTLinearDensityFit fit validity check isn't accurate

[evzayas]

A fit is performed on the rotate-and-projected spectrum in KTLinearDensityFit, and a parameter fIsValid is set to record whether the fit converged successfully. This boolean is determined by:

TFitResultPtr::IsValid()

but it doesn't always accurately describe the success of the fit. Sometimes fits with STATUS=FAILED or similar obviously bad results still return true, and contaminate what we expect are good fits with nonsense. Perhaps a different method of validating the fit should be used.