@falkamelung noted a race condition in #106 due to non-atomic directory
checking followed by os.makedirs. This pattern exists in many places
in the codebase so I went ahead and replaced the rest of them.
The new exist_ok usage should be terser and more idiomatic.
Add prepSlcALOS2.py based on prepRawALOS.py for ALOS2 stripmap SLC data. The main difference is using get_ALOS2_name() to grab unique string like "ALOS2210402970" instead of using get_ALOS_ALP_name() to grab string of "ALPSRP075780620" for ALOS PALSAR.
Bugs fixed for unpackFrame_ALOS2.py:
+ bug fix in unpack() while searching for image and leader files
+ add -p / --polarization option to be able to specify the polarization, with default value of HH, following the style of unpackFrame_ALOS_raw.py
Zhang Yunjun
Ryan Burns