Fix/update documentation

[GMMDMDIDEMS]

Due to #211 and the included commit https://github.com/ataitler/pyRDDLGym/pull/211/commits/58f3517572221e61bd1618f9b9b097a5155222c5 the documentation is not correct on how to import an agent.

Files affected:

• README.md
• docs/start.rst
• pyRDDLGym/Examples/Traffic/Experiments/eval.py
• pyRDDLGym/tests/test_basic.py
• pyRDDLGym/tests/unit_test_mars_rover.py
• pyRDDLGym/tests/testGymDrone.py
• pyRDDLGym/tests/seeding/Seedtest.py
• pyRDDLGym/tests/unit_test_power_gen.py

The tests folder looks outdated. I have made minimal changes so that it is possible to run the files. However, for pyRDDLGym/tests/testGymDrone.py I did not know to which domain it belongs (domain/instance path still invalid), for pyRDDLGym/tests/unit_test_mars_rover.py and pyRDDLGym/tests/unit_test_power_gen.py I get a KeyError.

[mike-gimelfarb]

Thank you, these are indeed much needed fixes.