Mutable/Redundant class variables

[kngwyu]

Currently, many classes of rlpy3 have many mutable class variables like Domain.
Apparently, this is a bad manner because it means we cannot create multiple instances of the class.
Or, we have also many cases where we override the class variable in __init__. In these cases, class variables are redundant.
So we should fix it so that

• All class variables are immutable
• No override for class variables
• Immutable class variables should be UPPER_SNAKE_CASE for noticeability