Add name argument to all cost functions

[HDembinski]

Closes #941

So far the iminuit.cost functions only support models that pass each parameter as a positional argument. This makes it difficult to use models whose number of arguments should be easy to vary.

To make working with the latter easier, the cost functions now support models with a variable number of arguments in the form def model(x, *par). The actual number of arguments and their names are then passed with the new keyword name, which can also be used to override the model parameter names found by inspecting the model signature.

• Template
• BinnedNLL
• ExtendedBinnedNLL
• LeastSquares
• UnbinnedNLL
• ExtendedUnbinnedNLL