Wrong NaN check in satin/helper_functions.py

[ch-k]

Hi Martin,
I've experienced some trouble when reading HRIT in conjunction with trollduction and area definitions of different extent.. I think in helper_functions.py the NaN-check in function boundaries_to_extent should be fixed.

for i in range(4):
    if extent[i] is np.nan:
        extent[i] = default_extent[i]

should be replaced with

for i in range(4):
    if np.isnan(extent[i]):
        extent[i] = default_extent[i]

Best regards,
Christian