Add functionality to read .nc files back into rsgpr

[erikmannerfelt]

When trying to read an .nc file (the output of a previous rsgpr run), an ugly panic occurs. Following #7, this should not result in a panic but instead a helpful error message. Also, reading an rsgpr output as an input is good for a few reasons:

• Read the information and processing log of an output. The easiest way right now is to use xarray in python, which involves a few lines. It would be much nicer to have this built in to rsgpr.
• Run more steps on a processed file. For example, this may be useful if the user wants to store intermediate files. Then, one can run rsgpr -f ... --steps "step1" -o step1.nc followed by rsgpr -f step1.nc --steps "step2" -o step2.nc"

For reference, here is what trying to retrieve the information about an output looks like now:

❯ rsgpr -f DAT_0122_A1.nc -i  
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: "File found but no '.rad' file found: \"DAT_0122_A1.rad\""', src/cli.rs:190:6
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace