ERS stack processing

[yjzhenglamarmota]

I have been processing ERS1/2 data from ESA (http://esar-ds.eo.esa.int/socat/SAR_IMS_1P/) and thought it would be helpful to add the following script to stripmapStack before we can run stackStripMap.py for ERS data.

unpackFrame_ERS_ENV.py

I have also updated ODR.py because the updated Delft orbits have some format changes, and the original hard-coded line https://github.com/yjzhenglamarmota/isce2/blob/7fcf2c5662627842a788e5954778d98e2b3ca3e7/components/isceobj/Orbit/ODR.py#L185parser no longer works. The updated version work for both the old arclist files and the new ones.