Adding continuous effects to durative actions

[Alelafar]

I added the decrease_continuous_effect and increase_continuous_effect functions in order to have a continuous change effect in a durative action and I added them also as two new ProblemKind.

Moreover, I suitably modified the PDDL Writer in order to dump the new tool to the PDDL domain.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 89.97429% with 39 lines in your changes are missing coverage. Please review.

Project coverage is 85.23%. Comparing base (c9bb278) to head (e3813bf).
Report is 3 commits behind head on master.

Files	Patch %	Lines
unified_planning/io/pddl_reader.py	61.01%	23 Missing ⚠️
unified_planning/model/action.py	76.92%	9 Missing ⚠️
unified_planning/model/effect.py	66.66%	4 Missing ⚠️
unified_planning/io/pddl_writer.py	96.22%	2 Missing ⚠️
unified_planning/model/problem.py	96.42%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #563      +/-   ##
==========================================
+ Coverage   85.19%   85.23%   +0.04%     
==========================================
  Files         201      201              
  Lines       27138    27451     +313     
==========================================
+ Hits        23120    23398     +278     
- Misses       4018     4053      +35     

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