PDDL Case sensitivity

### Subject of the issue
PDDL is not case sensitive, but the `pddl` library is.

### Your environment
- Package Version [0.4.6]

### Steps to reproduce
```py
from pddl.core import Predicate
x = Predicate("case")
y = Predicate("CASE")
z = Predicate("CASE")
print(x == y) # False
print(y == z) # True
```

### Expected behaviour
The library should not be case sensitive when comparing PDDL elements.

### Actual behaviour
See code snippet above.