Revamped the functionality of the 'stats' tool. I removed some subsetting options that I had used experimentally. Now the calculation for the statistics is based on the same library functions that are used in asf_export.

Fixed a bug in asf_export that prevented the default setting for passing the sample mapping properly.

The proper parsing of stats through write_hdf5_xml is still work in progress.
Merge branch 'roipac' of https://github.com/rudigens/ASF_MapReady into roipac
Worked out the statistics part of write_hdf5_xml. Passed the initial testing.