Add National Land Cover Database (NLCD) dataset

[nilsleh]

This PR adds the NLCD dataset, with selective download and using masks for desired years.

Dataset format:

• single channel .img file

Dataset features:

• 16 classes for US
• remapped the class labels to ordinal labels

Example:

Note:
If the year selection scheme is approved, I would open another PR to change CDL accordingly, so that users don't have to download data for all years.

[adamjstewart]

Plot looks great! Also need to add to docs.

[adamjstewart]

Missing test coverage for some lines: https://app.codecov.io/gh/microsoft/torchgeo/pull/1244/blob/torchgeo/datasets/nlcd.py

[nilsleh]

Missing test coverage for some lines: https://app.codecov.io/gh/microsoft/torchgeo/pull/1244/blob/torchgeo/datasets/nlcd.py

not sure why it is not working, I thought the following test would cover it but doesn't:

    def test_already_downloaded(self, tmp_path: Path) -> None:
        pathname = os.path.join(
            "tests", "data", "nlcd", "nlcd_2019_land_cover_l48_20210604.zip"
        )
        root = str(tmp_path)
        shutil.copy(pathname, root)
        NLCD(root, years=[2019])

[adamjstewart]

What if you add an additional test that includes years that don't exist locally and need to be "downloaded"?

[nilsleh]

found my mistake, and tested again with downloading the real data from scratch and retrieving samples.