An assembly plan, as defined in (SEP 055)[https://github.com/SynBioDex/SEPs/blob/master/sep_055.md], is an Activity with a certain structure, pointing to a Component that describes the plan following a particular pattern. We need a validation function that, given an assembly-plan object, checks to see whether it actually follows that pattern.

Note that this excludes checking whether the sequences for scars and restriction sites is correct. That will be handled separately. This is just a syntactic check to make sure that there are the right sorts of reactions and types to describe an assembly.