Refactor custom op system (#245)

* [Refactor] use getCustomOp instead of direct registry access

* [Refactor] move HLSCustomOp base to own file

* [Refactor] register all HLSCustomOps in new style

* [Refactor] use correct domain for custom ops acc. to new style

* [Deps] update finn-base to get new-style customop domains

* [Refactor] more domain fixes

* [Test] fix ipstitch expected io values in rtlsim

* [Deps] update finn-base and brevitas

* [Docs] link to CustomOp reorg PR