NCCM: fix timestamp extraction

[adamjstewart]

We weren't properly extracting the timestamp from the filename, we were just using the default 0 and sys.maxsize.

Before

> python3 -c 'from torchgeo.datasets import NCCM; print(NCCM("tests/data/nccm"))'
NCCM Dataset
    type: GeoDataset
    bbox: BoundingBox(minx=115.483402043364, maxx=115.48627665227318, miny=53.528522711204424, maxy=53.531397320113605, mint=0.0, maxt=9.223372036854776e+18)
    size: 3

After

> python3 -c 'from torchgeo.datasets import NCCM; print(NCCM("tests/data/nccm"))'
NCCM Dataset
    type: GeoDataset
    bbox: BoundingBox(minx=115.483402043364, maxx=115.48627665227318, miny=53.528522711204424, maxy=53.531397320113605, mint=1483225200.0, maxt=1577833199.999999)
    size: 3

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