Add checkpoint file as checkpoint, remove --force option

[joecummings]

No description provided.

[codecov]

Codecov Report

Merging #87 (0a1bd81) into master (f75423f) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master      #87   +/-   ##
=======================================
  Coverage   83.00%   83.00%           
=======================================
  Files           9        9           
  Lines         500      500           
=======================================
  Hits          415      415           
  Misses         85       85           

Impacted Files	Coverage Δ
pegasus_wrapper/pegasus_utils.py	100.00% <ø> (ø)
pegasus_wrapper/workflow.py	96.81% <100.00%> (ø)

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[lichtefeld]

lgmt!