make a lot of the same changes as to fs.py for checkpointing.

make a lot of the same changes as to fs.py for checkpointing.

1) rearrange the options to be in a nice logical order
2) add an option for what i call "standard switch", which is from simple->timing->detailed
3) make checkpointing code such that checkpoints taken from the command line override checkpoint instructions compiled into binaries.
4) add an option for maximum number of checkpoints - simulation will stop at max or maxtick, whichever is first

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