Update FOL Grammar Parser for Specifying Safety Properties

The first-order logical specification language is work in progress and a couple of further modifications are needed. The grammar itself is written in Antlr:

Some of the problems are displayed here:

# Changes to be made
- Use '=' instead of '==' to represent equality
- Introduce function symbols. In particular, mathematical functions like abs, min, max, sin, and cos should be allowed...
- It should be allowed to assign a wildcard to variables: ```a = *```, meaning that a can have an arbitrary value between ```-inf and inf```
- Function symbols are already possible, but a visitor should check whether specific symbols are allowed?