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.