Standalone SNAPHU unwrapping

[vbrancat]

This PR introduces a script to run SNAPHU unwrapping as a standalone module within ISCE3. The script can be used with the current version of the develop branch.

The script can be run as:
python unwrap_snaphu.py -r path2YAMLrunconfig

The runconfig can be built using options being exposed here: https://github-fn.jpl.nasa.gov/vbrancat/isce/blob/snaphu_insar_work_branch/share/nisar/schemas/snaphu.yaml
An example of a runconfig file as well as a copy of the schema can be included in this PR.

It is the user responsibility to assign nlooks (effective number of looks) to properly run SNAPHU. The script provides functionalities to compute nlooks based on basic info but cannot compute it automatically

[yunjunz]

@vbrancat There is a new public release on isce3 version 0.5 (https://github.com/isce-framework/isce3/tree/v0.5.0). Could we use that for this script?

[vbrancat]

@yunjunz yes, that is the version of ISCE3 containing the SNAPHU Unwrapper.

[hfattahi]

@vbrancat thanks for adding this stand-alone script. Would you please add the schema and example yaml so that we can test it?

[hfattahi]

I don't see this used here.

[vbrancat]

snaphu_schema.txt
snaphu_default.txt

Schema and default are attached here.

[vbrancat]

I realized that I was not checking the schema so I have introduce a step to load the runconfig and validate it against the schema. I also updated the code to include the selection of the initialization method since MCF has been now merged in ISCE3 :D