added save/load gridmap for GridTransitionMap

1 job for master in 2 minutes and 8 seconds (queued for 6 seconds)
Name Stage Failure
tests Test
make: *** [coverage] Error 1
ERROR: InvocationError for command '/usr/bin/make coverage' (exited with code 2)
___________________________________ summary ____________________________________
ERROR: py36: commands failed
ERROR: py37: commands failed
ERROR: flake8: commands failed
docs: commands succeeded
ERROR: coverage: commands failed
ERROR: Job failed: command terminated with exit code 1