Negative density in MALA

[cpfiffer]

Hi all. I recall that minus sign... Let me take a look. Yes perhaps within an issue is more appropriate. I will be a little slow though - I am deep in an academic search right now and about to start interviewing, very time intensive and stressful. But hopefully won't be months, like my original PR :)

Originally posted by @briandepasquale in https://github.com/TuringLang/AdvancedMH.jl/pull/58#discussion_r640619695

Seems there's a negative sign in the MALA code -- not sure why.