StereoMatcher extraction, building StereoInput

...time for a commit again. I hate committing unfinished stuff but at
least this compiles and runs (although it's not using StereoInputs yet).
I know, I should be committing daily, not monthly...

So, most of the StereoMatching business has been encapsulated in the
StereoMatcher class. It's not finished yet, the input image processing
pipeline is still sort of happening outside the class, but key methods
and members have been conglomerated inside and should allow
multi-instanciation (haven't tested that though).

Work is progressing on the StereoInput class, which will encapsulate any
kind of stereo input (videos, static images etc.), including loading
from files.
I have decided to use zip archives for this purpose; stereo matching
parameters are stored in a JSON dictionary/string for convenience.

PointSampler has also been renamed to TreeSampler and made generic.