Parsed problem is inconsistent with its creator

[DianLiI]

Describe the bug
p2 = PDDLReader().parse_problem(PDDLWriter(p1).get_domain(), None) can produce a p2 that's different from p1.

As a matter of fact, it seems like anything defined with (:function) is always parsed as real fluent regardless of its usage.

To Reproduce

from unified_planning.shortcuts import *
from unified_planning.io import PDDLWriter, PDDLReader

Counter = UserType('Counter')
value = Fluent('value', IntType(), m=Counter)

inc = InstantaneousAction('increment',c=Counter)
c = inc.parameter('c')
inc.add_precondition(LE(value(c), 10))
inc.add_increase_effect(value(c), 1)

problem = Problem('problem')

problem.add_fluent(value, default_initial_value=1)
C0 = Object('c0', Counter)
problem.add_object(C0)
problem.add_action(inc)
p2 = PDDLReader().parse_problem_string(PDDLWriter(problem).get_domain())
print("Original problem:")
print(problem.kind)
print("Parsed problem:")
print(p2.kind, "<-- this is different")

Output:

Original problem:
PROBLEM_CLASS: ['ACTION_BASED']
PROBLEM_TYPE: ['SIMPLE_NUMERIC_PLANNING']
EFFECTS_KIND: ['INCREASE_EFFECTS']
TYPING: ['FLAT_TYPING']
FLUENTS_TYPE: ['INT_FLUENTS']
Parsed problem:
PROBLEM_CLASS: ['ACTION_BASED']
PROBLEM_TYPE: ['SIMPLE_NUMERIC_PLANNING']
EFFECTS_KIND: ['INCREASE_EFFECTS']
TYPING: ['FLAT_TYPING']
FLUENTS_TYPE: ['REAL_FLUENTS'] <-- this is different

Expected behavior
problem -> PDDLWriter -> problem_str -> PDDLReader -> problem should not create discrepancies.

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