Typo of PDDL writer for 'at' predicate in problem file

[LamannaLeonardo]

The PDDL writer (v1.2.0) seems to introduce a typo when writing a PDDL problem file involving a unary predicate named at, e.g. (at c0) is written as (at_ c0).

Here is a code snippet to reproduce the issue:

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

problem = Problem("test")
obj_type = UserType("object")
at = Fluent("at", BoolType(), obj=obj_type)
problem.set_initial_value(at(Object('c0', obj_type)), True)
PDDLWriter(problem).write_problem('prob.pddl')