Rebuilding generated protobuf code

If you make changes to .proto files, you will need to rebuild the generated Go code. To do that, you will need:

• The protobuf compiler
• A proper Go development setup per BUILD.md (variables GOPATH and GOBIN set properly)
• go get -u github.com/golang/protobuf/protoc-gen-go

If all this setup is complete, simply run go generate in the current directory (pkg/beam/model).