fix a tiny bug

two level checkpointing works perfectly; stupid revolve!

reduce number of checkpoints (the first and last ones are removed)

refactor the code

change some names

add two-level checkpoiting, the version without revolve works

Postpone stack allocation to setup stage of tstrajectory, -tstrajectory_max_cps works now

trying to add an option -tstrajectory_max_cps

refactor the code a little bit

fix the memory access bug when creating revolve object

cleanup trailing whitespaces

forgot to add revovle source files

results verified to be correct

allow recomputing the solution when not all time steps are checkpointed in memory
for testing, the checkpoints are randomly determined

clean up spaces after comma

Use PetscFunctionBegin instead of PetscFunctionBeginUser

fix a bug related to stack size; allow overwriting top elements in stack

bug fix for checking stack overflow

add in-memory checkpointing, and it works :)

correct more names for TSTrajectory and add TS to the argument list when creating TSTrajectory

use tj to name a TSTrajectory variable

fix compile errors

merge nek-style checkpointing here mannually

Merge branch 'hongzh/ts-adjoint-monitor'

refactor the code to prepare for all the three versions of revolve