Add support to TILs in the PDDLWriter

[alvalentini]

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[codecov-commenter]

Codecov Report

Attention: 6 lines in your changes are missing coverage. Please review.

Comparison is base (da8ab70) 84.96% compared to head (51f5adc) 85.00%.
Report is 11 commits behind head on master.

Files	Patch %	Lines
unified_planning/io/pddl_reader.py	66.66%	5 Missing ⚠️
unified_planning/io/pddl_writer.py	96.77%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #543      +/-   ##
==========================================
+ Coverage   84.96%   85.00%   +0.03%     
==========================================
  Files         200      200              
  Lines       26433    26469      +36     
==========================================
+ Hits        22458    22499      +41     
+ Misses       3975     3970       -5     

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[arbimo]

LGTM. I just suggested using timed-initial-effect has feature name to make it less ambiguous and avoid potential clashes.

Right now there is no place for it in the documentation but it should be mentioned in a future "advanced temporal modeling features".