recursive -> degopt

[jarlebring]

As agreed in the meeting, what is currently called general recursive form should be changed to degree optimal, so

• All functions ending with _recursive should be changed to _degopt.
• The gen_general_recursion should be changed to gen_degopt,
• Similar for function documentation

[eringh]

2bc92df and ea3db50 cover the name change. I also think they cover the documentation.