track total ll for trace
actually memoize in MH mode
fix behavior on mismatches, etc
remove dead code
add {MAP,MLE}-estimate methods to mh-sampler

Also add linear regression example.
enable normal custom proposal by default
fix trace datatype, record free choice ll vs obs ll separately
optimization: omit obs entries from db (safe if they don't change)

For the linear regression example with 100 observations, the
optimization cuts the time about in half.