Suggest to update fetchOrbit.py and topsStack README.md

[yuankailiu]

fetchOrbit.py:

• Allow reading SLC zip files from a given folder via -d option. So fetchOrbit.py can be used directly as a script to download a batch of orbit files by specifying the SLC folder, rather than doing -i for each input *.zip file.

topsStack README.md:

• The run_files descriptions in README.md of the topsStack processor are outdated. Here the proper names of each run file is used. Some brief explanations are also updated for each run file.

[chintals]

Great edit! One brief comment (not related to this PR), the commands in run files for ionospheric estimation are all in serial order (except run_ns+1, subband_and_resamp) regardless of nproc setting. Curious as to why the ns+ files are not parallelized with nproc setting?

[yuankailiu]

I don't know. I have always been using HPC cores to do the parallelization of each command line in every run file with stackSentinel.py --num_proc 1.