Update comments and docstrings (add Raises)

Final changes (docs and README update) before 2.0 release
Closes #97 (closed) #121 (closed)

Also a big docstrings update is required for the API docs.

Other changes :

• removed one old corner case for OTB<7.4 in functions.py 50a51fd2
• factorize one duplicated function: a21aa4df
• light refac in functions.py but no change to the logic : e670786a
• ensure both list or tuple is are accepted for list parameters value
• add an exception is __set_param in case a list is passed to a non list key 848f4e4a