FILE IO
1. utils/writefile.py
1.1. remove sorting argument in write_roipac_rsc()
1.2. make LENGTH key optional in write_roipac_rsc()

2. utils/readfile.py
2.1 add ISCE metadata key to global variable standardMetadataKeys
2.2 add new global variable ENVI2NUMPY_DATATYPE and make it together with GDAL2NUMPY_DATATYPE in string for both key and value
2.3 add new function read_binary() for generic binary data file reading, to replace all obsolete functions like read_float32(), read_real_float32(), etc.
Those obsolete functions will still be kept for old users; but are not used in PySAR code since now.
2.4 adjust and rename attribute_envi2roipac() to standardize_metadata_isce()
2.5 add PAMDataset xml file support in read_isce_xml()
2.6 simplied read_attribute() with new argument "meta_ext" to specify the correponding metafile with a pre-fixed extension, used in new script prep_isce.py
2.7 adjust read_binary_file() to use the new generic read_binary() instead of all those small functions.

3. add new script - prep_isce.py, modified from ISCE/SentinelStack code with the following changes:
3.1 use readfile.read() instead of gdal for file reading, because gdal is hard to install
3.2 write master/IW1.xml file extraction result into data.rsc file, to avoid loading ISCE module eveytime when prep_isce.py is called.
3.3 move load_product() from s1a_isce_utils.py to make it independent from topsStack code
3.4 add new function extract_multilook_number() to grab A/RLOOKS
3.5 simplied prepare_geometry/stack()
3.6 use pysar.utils.readfile module to all xml/rsc file reading/writing except for the IW1.xml file reading and conversion

4. support prep_isce.py in load_data.py
4.1 add new template options for ISCE only
pysar.load.metaFile
pysar.load.baselineDir
4.2 adjust prepare_metadata() to call prep_isce.py, so that pysarApp.py can be run directly after topsStack processing, without calling prep4timeseries.py
4.3 simplified defaults/auto_path.py