New algorithm keyword argument to contour and contourf

The contouring functions contour and contourf have a new keyword argument algorithm to control which algorithm is used to calculate the contours. There is a choice of four algorithms to use, and the default is to use algorithm='mpl2014' which is the same algorithm that Matplotlib has been using since 2014.

Other possible values of the algorithm keyword argument are 'mpl2005', 'serial' and 'threaded'; see the ContourPy documentation for further details.