Support subtraction in preconditions in domains.

**Is your feature request related to a problem? Please describe.**

The DomainParser cannot seem to handle `(- (function_name object_name) 1)` as part of a precondition.

Here is an example domain showing this issue. (In this domain, the arithmetic in the precondition isn't needed since we compare to a constant `1`, but in my actual domain of interest we compare to another numeric function).

```pddl
(define (domain cant-subtract-in-precondition)
(:requirements :equality :typing :fluents :negative-preconditions :universal-preconditions :existential-preconditions)

(:types
agent - object
)

(:functions
(x ?l - agent)
)

(:action move-south
:parameters (?ag - agent)
:precondition (= (- (x ?ag) 1) 1)

:effect (and (decrease (x ?ag) 1))
)

)
```

**Describe the solution you'd like**
Parse the subtraction correctly.

**Describe alternatives you've considered**
As a workaround, I can do `(+ (function_name object_name) (- 1))`

**Additional context**
Add any other context or screenshots about the feature request here.