Merge pull request #23 from rg2/example_build_improvements

updates to support compiling with opencv 4+

* disabling use of eigen for opencv builds,

changing example build script do GUI variable to be set via environment variable

Merge pull request #19 from rg2/sampling

fixing a bug that with disabling prior by default

changing the default prior params to be more sensible

adding a convenience flag to discard prior

handing rotation to "up"

adding downsampling factor as command line argument

adding specification of prior params on command line

initial impl. of multivariate normal approx for likelihood

initial implementation of a dist. and sampler for product of

adding a getter for the inverse covariance matrix stored for a

rearraging the data I/O code to top of file