Add a findfit function

[fdekerme]

Since the package is defined as "a simple interface for users who want to quickly run standard parameter estimation routines for model calibration", it would be interesting to add a findfit function on the model of the homonymous functions in Wolfram Mathematica (https://reference.wolfram.com/language/ref/FindFit.html ; https://community.wolfram.com/groups/-/m/t/126143) or SageMath (https://doc.sagemath.org/html/en/reference/numerical/sage/numerical/optimize.html#sage.numerical.optimize.find_fit) for example. This findfit function would combine a "generic" cost function, the definition of the OptimizationProblem and the solving.