Cleaner check of the number of experiments

[fretchen]

Right now we are checking the number of experiments in an utterly obscure fashion namely through:

try:
            exp_ok = (
                expr.startswith("experiment_")
                and expr[11:].isdigit()
                and (int(expr[11:]) <= max_exps)
            )
        except:
            exp_ok = False
            break

It is completely impossible to know what is going on there...

[RohitPrasadBhatt]

So:

• expr.startswith("experiment_") checks if the experiment name starts with "experiment_".
• The other two conditions check if the suffix is a valid digit in correct range.

If any of those checks fail due to mismatch or an exception the boolean flag is set to false.

Maybe regex can make it a bit slim. Something like ^experiment_[0-9]+$. Caution : verify the expression before using it and set bounds for maximum experiments properly.