DRY Experiment#initialize

[pakallis]

Experiment#set_alternatives_and_options appeared both in the if
section and in the else section. I moved set_alternatives_and_options
out of the if...else block so that it is more DRY.

I am not sure if setting only options[:alternatives] in the else
clause is more readable than explicitly providing a hash with the attributes
to set as it happens in the if clause.

[andrew]

I like it, thanks @pakallis 👍