Refactor stereopair metadata parsing and geometry plot for more flexibility

[bpurinton]

On #24, the work done to extract scene metadata and plot the geometry (sky plots) was highly specific to WV scenes. Really, it should be written more flexibly for ASTER, HiRISE, etc., etc.

This is a hairy problem and requires a case-by-case study of metadata for each product. Likely this will need to be done iteratively at a later point after knocking out some of the other issues on the board here.

[bpurinton]

Actually, why re-invent the wheel for XML parsing? We should really just reuse the methods of ASP, like shown in this file:

https://github.com/NeoGeographyToolkit/StereoPipeline/blob/master/src/asp/Camera/RPC_XML.cc

All of the camera parsing is done already by ASP so we should extract relevant, useful details and output a report / table directly from ASP during processing. Then the code here can slurp that report which will be standardized for any given camera / sensor.

Alternatively to ASP outputting this report, can we call the methods of interest here?