Implement __call__ for Grid instances

Grid instances should be a callable on a given function; calling an instance of Grid with a function/callable should evaluate the given function on the unisolvent nodes and return the corresponding function values (i.e., the Lagrange coefficients).

The function/callable that can be passed as an argument to an instance of Grid shall accept as its first argument a two-dimensional NumPy array because the unisolvent nodes are always given as a two-dimensional NumPy array. Additional arguments to the given function may be passed as __call__() also accepts *args and **kwargs that are propagated to the function.

The current codebase has a not-yet-implemented method called apply_func() which would be less convenient than directly call the instance.

This issue is part of the refactoring of the Grid class (Issue #135).