Categorizing / classifying and specifying work and services.

Capturing from https://github.com/valueflows/valueflows/pull/296#discussion_r236713077

@elf-pavlik I would not use ResourceSpecification since we don't reference any resource from work flows and this would just make everything more confusing. Only flows which at event stage will reference an actual resource with affects should use vf:resourceConformsTo and reference specification of a resource which event will reference using vf:affects.