Static layers: close the scratch rasters before removing the scratch directory

[lijun99]

The static layers workflow writes its intermediate layers as GeoTIFFs in the scratch directory and keeps them open (isce3.io.Raster objects) until run_static_layers_workflow returns, i.e., after scratch_directory has removed the directory with shutil.rmtree.

• On local file systems, removing open files succeeds.
• On NFS, the client renames an open file that is removed to a hidden .nfsXXXX file, so the directory is not empty and rmtree fails with OSError: [Errno 39] Directory not empty: 'scratch'.

The same NFS behavior was reported in #207 for the scratch directory of nisarqa, when running the RSLC and InSAR PGEs with the working directory on NFS (at HECC, and on the on-demand systems). So the static layers workflow can fail in the same environments.

This PR closes these rasters with Raster.close_dataset() at the end of the scratch_directory block, after they are written to the product, so the files are closed before the directory is removed.

Testing: tests/python/packages/nisar/workflows/static.py was run from a directory on NFS.

• Before: the first test case failed, leaving 8 .nfsXXXX files in scratch, one for each open raster. The second case passed only because scratch then existed and was not removed.
• After: both cases pass, and scratch is removed.