refactor the Experiment class to be more user friendly

Experiment class code refactoring:

• new API (getters, setters, run_once(), run(rounds()) with near-backward compatibility
• small changes in API (rounds => round_limit, getters syntax eg model_instance => model_instance())
• robustify code with argument checking
• robustify code with testing/catching errors in code
• better exception handling (propagate #212 (closed) to Experiment class + prettier messages for user