Pulling from the `multi-band-geotiff` branch from my personal fork, that is currently in a pull request at image-rs/image-tiff#224.

Count the number of bands or channels in the GeoTIFF file when decoding by looking at the ColorType. E.g. Gray = 1 band, Multiband = N bands. Added a unit test to read a 2-band float32 GeoTIFF file.

Ensure that reading a multi-band GeoTIFF file from a remote URL works.

Also clarify that the roadmap item on reading single-band GeoTIFFs is for different dtypes.

The Multi-band PR at image-rs/image-tiff#224 has been merged, so updating to use the main branch instead.

Replace `unimplemented!` with a recoverable TiffError::UnsupportedError instead to allow for better exception handling. Moved the band counting logic in the ndarray method above the image dimension line, and added a unit test to ensure unsupported ColorType TIFFs are handled properly.