Check that CDFs are non-decreasing - continuous_ranked_probability_score

[mattmotoki]

I think it is important to check that the predicted CDFs are non-decreasing. Without this requirement, the continuous_ranked_probability_score can produce incorrect results.

>>> import numpy as np
>>> from solarforecastarbiter.metrics.probabilistic import continuous_ranked_probability_score
>>> continuous_ranked_probability_score(np.array([0.5]), np.array([[0.5, -1e6, 0.5]]), np.array([[0, 0.5, 1]]))
-9950.004974999989