Some refactoring of nephron development, spurred by #438

Some work remains to be done. I have deviated from the
ZFA pattern whereby primitive nephrons are subtypes of nephrons,
instead opting for a future-X-->X type pattern.

However, there is some remaining confusion about progenitors vs primitive.

Need to examine this ZFA pattern in more detail:

```
[Term]
id: ZFA:0005587
name: nephron progenitor
namespace: zebrafish_anatomy
def: "Cluster of cells comprising a portion of tissue which gives rise to new mesonephric nephrons.  The cluster contains self-renewing stem cells." [ZFIN:ZDB-PUB-110131-33]
xref: ZFIN:ZDB-ANAT-110523-9
is_a: ZFA:0001477  ! portion of tissue
relationship: end ZFS:0000044 ! Adult
relationship: part_of ZFA:0000529 ! kidney
relationship: start ZFS:0000000 ! Unknown

[Term]
id: ZFA:0005584
name: developing mesonephric nephron
namespace: zebrafish_anatomy
def: "Anatomical cluster which give rise to mature mesonephric nephrons.  Zebrafish continously generate new mesonephric nephrons." [ZFA:curator]
xref: ZFIN:ZDB-ANAT-110523-6
is_a: ZFA:0005282  ! nephron
relationship: end ZFS:0000044 ! Adult
relationship: start ZFS:0000000 ! Unknown
```
olfactory epithelium DF prim. olf. epithelium PO olf. placode. todo: other parts of placode.Issue #438
Defs for future germ layers. Issue #438