rewrite logcombine

The routine is much more targeted in terms of where the transformation
takes place and also combines logs that have a common coefficient, e.g.

x*log(3) + x*log(2) will now become x*log(6)

This only works if the coefficient is the same on both terms otherwise,
finding pairs to combine becomes costly.
remove all expansion in logcombine
fix failing ode doctests and tests
-log(x) -/-> log(1/x); check to put neg log's arg in denom

It's a matter of aesthetics ... I prefer to keep the -1 out in front of
the log. If there are others that it would join with, this doesn't matter
since log(x) - log(y) -> log(x) + log(1/y) -> log(x*1/y) -> log(x/y)
But when forming an exponent, log(-log(y)) looks better, I think, than
log(log(1/y)).
add note about logcombine
Merge pull request #1845 from smichr/logcombine

logcombine modifications