Test contour levels that are +/-np.inf

[ianthomas23]

Adding tests to confirm that contour levels of -np.inf and np.inf are valid on all platforms. These are useful, for example, to produce filled contours from a level z upwards which would be achieved using

from contourpy import contour_generator
import numpy as np

cg = contour_generator(...)
filled = cg.filled(1.0, np.inf)

Doc update with examples to follow in a separate PR.