Add validate method to FisherDerivative

[JCGoran]

It's possible that a user makes a typo on some parameter when calling, say, fisher_matrix, and therefore the whole computation may fail, wasting valuable time.

Proposal

Add a method called validate (or is_valid), which by default returns True, and which a user can override if they want to perform validation of the parameters before doing any computation.
As fitk only has complete control over the derivative and fisher_matrix methods, it should be implemented at that level, rather than in signal or covariance.