Only allow dumping and loading checkpoints in freeze mode

[jdeans289]

If you dump or reload a checkpoint while not in freeze mode, you can have unexpected results. Currently, the sim control gui enforces this by greying out buttons, but there is no mechanism preventing a checkpoint dump or reload in run mode in Trick itself, so manually calling dump_checkpoint in run mode can result in confusing bugs.

This issue can be closed when:

• Trick prints a warning and refuses to dump/load a checkpoint if not in freeze mode
• This behavior is tested and documented