Operator._prepare_arguments needs refactoring to remove Thickness loop

[EdCaunt]

Operator._prepare_arguments currently contains the following lines:

# Process Thicknesses
for p in self.parameters:
    if isinstance(p, Thickness):
    args.update(p._arg_values(grid=grid, **kwargs))

which could be removed by making Thickness have is_Input = True and moving grid processing to before _arg_values is called on inputs.

However, as pending PRs make changes to this code, this should be returned to later to avoid messy merge conflicts.