When k > 0, use symmetrical rate function of k < 0
Doc: Add page with time-flip proposal background
Merge branch 'new-rate-function' into time-var-const
Create TimeModeProposal as a base class

LambdaTimeModeProposal now derives from TimeModeProposal and was
updated appropriately. BetaTimeModeProposal will also derive from
TimeModeProposal in the future.
Doc: Actual description of time-flip is here

A previous commit only added the entry to the contents page, but it did
not contain the time-flip.rst file.
Add time-flip proposal for beta