VASP IO copy() methods

[janosh]

as pointed out by @esoteric-ephemera, calling .copy() on VASP IO classes like Incar and VaspInput returns a dict rather than a new instance of the class as most users would expect. this PR adds explicit copy() methods to minimize user surprise.

@esoteric-ephemera that mixin snippet i sent doesn't work as the __init__ signatures of IO classes are too different. for the record, we considered defining

class CopyableMixin:
    def copy(self):
        return type(self)(self)

and making all Vasp IO classes inherit from it

[esoteric-ephemera]

Thanks @janosh! Question for @shyuep: @janosh and I were discussing if it makes more sense to define the self.copy method for MSONable - do you think it makes more sense to define the copy method for specific classes that inherit from MSONable, or for MSONable itself?

[shyuep]

Depends on how general the copy method is. If it is generic and uses the MSONAble capabilities to do the copy, then it makes sense.

[shyuep]

Just an addendum that when I say "makes sense", it means it makes sense for any MSONable class, not just VASP io related stuff.

[janosh]

might also make sense to turn all VASP IO classes into @dataclasses so we get __eq__ for free and copy with minimal variation