Add copyright notice to all source files

[ErC]

The Apache 2 license requires us to add the following notice to all source files:

Copyright 2021, Haveno

Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at

http://www.apache.org/licenses/LICENSE-2.0

Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
See the License for the specific language governing permissions and
limitations under the License.

We haven't included it in any file yet, but we should definitely do it in the future (and should be added to already existent files).