MES Factory gradient support

[BryceFuller]

Summary

Updating the MES Factory methods to support the new gradient framework.
This PR resolves #1398

Details and comments

[woodsp-ibm]

The existing parameters to the factories, such as expectation, all have setter/getters. I think for consistency gradient should have them too.

[BryceFuller]

The existing parameters to the factories, such as expectation, all have setter/getters. I think for consistency gradient should have them too.

good catch, thanks

[woodsp-ibm]

In looking at the setters/getters now that you have done these I notice the original ones for initial_point, expectation, optimizer that default to None in factory constructor do not allow show that None is a valid value to set these too, via setter, or one would expect to have None back from getter, ie. are Optional like you just did for gradient. I think it should be valid to set to None so you go can back to default. Either way the getter needs to show Optional. Maybe this should/could be done in a separate PR but since its just typehints maybe it can be added/fixed here?

Update: I created a new issue #1428 to address the above, leaving this as purely the gradient change then.