+ convert README.txt to README.md
+ stackStripMap.py: group iono options in createParser()
- group all iono options in createParser() for more easily navigation in --help.
- fix typos in the descriptions for the script and --dem option.
expose the -f / --filter_strength option in the parser to user so that one is able to change the filtering strength. Default is setted as the hardwired value - 0.8, so previous setting won't be affected.