Add LTLf syntax support.

[marcofavorito]

Types of changes

Add support for LTLf syntax and parsing:

• tt and ff
• atomic formula
• negation
• and
• or
• implies
• equivalence
• xor
• next
• weak next
• until
• release
• eventually
• always

Type of changes:

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov]

Codecov Report

Merging #2 (48eee5e) into develop (8212620) will increase coverage by 0.87%.
The diff coverage is 37.68%.

❗ Current head 48eee5e differs from pull request most recent head 4cf2daf. Consider uploading reports for the commit 4cf2daf to get more accurate results

@@             Coverage Diff             @@
##           develop       #2      +/-   ##
===========================================
+ Coverage    41.46%   42.33%   +0.87%     
===========================================
  Files           16       23       +7     
  Lines          123     1729    +1606     
  Branches        52      675     +623     
===========================================
+ Hits            51      732     +681     
- Misses          40      478     +438     
- Partials        32      519     +487     

Flag	Coverage Δ
cynthia-core	?
cynthia-logic	38.20% <36.21%> (-17.48%)	⬇️
cynthia-parser	42.33% <37.68%> (+1.01%)	⬆️
cynthia-utils	42.30% <38.09%> (-24.36%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
libs/logic/include/cynthia/logic/comparable.hpp	45.45% <ø> (-54.55%)	⬇️
libs/parser/include/cynthia/parser/parser_stype.h	100.00% <ø> (ø)
libs/parser/include/cynthia/parser/parser.yy	8.33% <9.52%> (ø)
libs/logic/src/ltlf.cpp	16.90% <18.03%> (-12.52%)	⬇️
libs/logic/tests/unit/test_ltlf.cpp	19.28% <19.15%> (-2.94%)	⬇️
libs/parser/tests/unit/main.cpp	37.24% <37.24%> (-62.76%)	⬇️
libs/utils/tests/unit/main.cpp	37.50% <38.46%> (-12.50%)	⬇️
libs/utils/include/cynthia/utils.hpp	46.42% <42.30%> (-53.58%)	⬇️
libs/logic/src/base.cpp	48.23% <48.14%> (+8.23%)	⬆️
libs/parser/include/cynthia/parser/driver.hpp	50.00% <50.00%> (ø)
... and 22 more

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[marcofavorito]

need to complete these!!

[Shufang-Zhu]

I have a question about tt and true.
I guess make_tt if for tt, what about true? Are we considering it as an atom?

[Shufang-Zhu]

I think I have asked this question.
How do we define symbol?