relabel zygote to 'animal zygote'

[cmungall]

   CL:0000548 ! animal cell
       is_a CL:0002321 ! embryonic cell
        is_a CL:0000007 ! early embryonic cell
         is_a CL:0000365 ! zygote

the term 'zygote' is used generically in plants

CL should either be generic, or the class should be relabeled

[cmungall]

On call @addiehl @ybradford @cerivs @nicolevasilevsky

We decided that we will relabel this to 'animal zygote', with broad syn 'zygote'

We will not add a new generic class now, though we can explore this in future if a use case arises (note we would need an inter-ontology subclass with PO)