Non indexable objects

[softmattertheory]

Describe the bug
Classes that don't provide index and setindex should raise errors if you try to index them.

To Reproduce
// check index
class A {
}

var b = A()
print b[0] // expect error

Expected behavior
Should raise an error; presently returns nil

Additional context
Add any other context about the problem here.

[softmattertheory]

Currently, all classes provide index and setindex because they inherit from Object. It may perhaps be useful to have an ImmutableObject class, but that feels like a separate issue.

But there is still a problem here:

print b["hello"] 

does raise a 'ObjLcksPrp' error.