This commit adds experimental CMake build support for ISCE2.
The only changes involved should be adding CMakeLists.txt files.
All current source files, headers, and other functionality, including
the existing SCons build system, should be unaffected.
Some functionality is still a work-in-progress.
These should all be marked as TODO in the CMakeLists.txt files.
`grep TODO **.txt`
Please read CMakeLists.txt and .cmake/*.cmake for more info.
Ryan Burns