Replace np.NaN by np.nan - Numpy 2.0 compatibility.

[Borlaff]

Hi,

In sky.py, class ZodiacalLight(Sky), _zl_scale should use np.nan instead of np.NaN for compatibilty with Numpy 2.0, since the former is deprecated.

    _zl_scale = np.array([[np.nan, np.nan, np.nan, 3140, 1610, 985, 640, 275, 150, 100], \
                          [np.nan, np.nan, np.nan, 2940, 1540, 945, 625, 271, 150, 100], \
                          [np.nan, np.nan, 4740, 2470, 1370, 865, 590, 264, 148, 100], \
                          [11500, 6780, 3440, 1860, 1110, 755, 525, 251, 146, 100], \
                          [6400, 4480, 2410, 1410, 910, 635, 454, 237, 141, 99], \
                          [3840, 2830, 1730, 1100, 749, 545, 410, 223, 136, 97], \
                          [2480, 1870, 1220, 845, 615, 467, 365, 207, 131, 95], \
                          [1650, 1270, 910, 680, 510, 397, 320, 193, 125, 93], \
                          [1180, 940, 700, 530, 416, 338, 282, 179, 120, 92], \
                          [910, 730, 555, 442, 356, 292, 250, 166, 116, 90], \w
                          [505, 442, 352, 292, 243, 209, 183, 134, 104, 86], \
                          [338, 317, 269, 227, 196, 172, 151, 116, 93, 82], \
                          [259, 251, 225, 193, 166, 147, 132, 104, 86, 79], \
                          [212, 210, 197, 170, 150, 133, 119, 96, 82, 77], \
                          [188, 186, 177, 154, 138, 125, 113, 90, 77, 74], \
                          [179, 178, 166, 147, 134, 122, 110, 90, 77, 73], \
                          [179, 178, 165, 148, 137, 127, 116, 96, 79, 72], \
                          [196, 192, 179, 165, 151, 141, 131, 104, 82, 72], \
                          [230, 212, 195, 178, 163, 148, 134, 105, 83, 72]]).transpose()