Missing Last, End special symbols in LTLf.

[cipollone]

Subject of the issue

Last, End are special symbols that should have a special meaning. After #18 , these are no longer ignored, but they are parsed correctly. However, the feature is still not supported. There is no LTLfLast class, and LTLfEnd must be tested. Writing last or end inside a LTLf parsed formula causes a NotImplementedError.

Your environment

• OS: any
• Python version: using Python 3.7.3, should apply to any.
• Package Version: no branch support this feature up to now (4/3/20)

[cipollone]

LTLfLast is now suppoted: #21.
We can leave this issue open, if we desire an LTLfEnd in the future.