remove tests/reference from the regular build

Because it had a SConscript file, it was getting picked up
by BasicSConstruct.initialize and built as part of an
"install".  This is not desirable, as it requires gfortran.
Renaming the SConscript to SConstruct removes it from the
build, but you can still go into that directory and run
"scons" to build it.
Split models.h into Model.h and MultiModel.h
Split priors.[h,cc] into Prior, MixturePrior, SoftenedLinearPrior
Rename constants.h to common.h
Refactor/rename sampling interface files to match LSST side.
Fix doc bugs to eliminate doxygen warnings.
Add documentation for Model, MultiModel, UnitSystem
Prune unused imports
Sync numpy.inf/Inf usage with LSST branches
Fix and test normalization of SoftenedLinearPrior
Renamed ProjectedLikelihood to UnitTransformedLikelihood
Delay throwing the exception when FitPsf fields aren't found in Schema

This allows us to create a CModel instance that knows how to copy from Results
to Records when we just want to pass a MultiShapeletPsf in directly.
Improve logging for TruncatedGaussian, but disable it at compile-time
Remove reference to now-nonexistent reference implementation SConscript
raise exception for area=0 region
Merge branch 'u/bick/HSC-1051'
Merge branch 'u/jbosch/HSC-1014'
Make "is there a solution" threshold smarter in TruncatedGaussian

The previous (generally tighter) threshold was frequently misdiagnosing singular
matrices with many solutions as problems with no solution.  The threshold is now
better tied to the size of the numbers being subtracted.
Add method to remove SR1 secant term from optimizer Hessian

Sometimes we want the Hessian without the SR1 term.  We'll just modify it in-place,
because when we want to remove it, we'll typically have already done the
optimization.
Remove the SR1 secant term from the Hessian before using it to compute errors

While the secant term is good overall, as it improves convergence, it can lead to
incorrect Hessians, at least for the linear parameters, where we can easily compute
the exact Hessian without it.