Add prM protein, remove PRO (pr) protein

The precursor protein is cleaved into ~10 proteins including the
precusor membrane protein (prM). prM is subsequently cleaved by the host
protease furin resulting in protein M (aka MP).

The previous annotation included CDSs PRO and MP, but not the (parent)
prM protein. prM is cleaved directly from the polyprotein and is
functional, forming heterodimers with protein E (similar behaviour
across all flaviviruses I believe).

NCBI's genome viewer shows both prM as well as both the sub-proteins
after cleavage, pr and M. pr would correspond to the previous PRO
annotation. I've not seen protein pr referenced anywhere else so I've not
included that here.

Note that this change wasn't desirable before Auspice could display
overlapping CDSs <nextstrain/auspice#1684>.

References:
<https://www.microbiologyresearch.org/content/journal/jgv/10.1099/vir.0.18723-0>
<https://www.ncbi.nlm.nih.gov/gene/7751225>

Author: jameshadfield

config/zika_outgroup.gb

@@ -100,9 +100,10 @@ FEATURES             Location/Qualifiers
      CDS             91..456
                      /product="capsid protein"
                      /gene="CA"
-     CDS             457..735
-                     /product="propeptide"
-                     /gene="PRO"
+     CDS             457..960
+                     /note="Modified by Nextstrain; Differs from ref"
+                     /product="precursor membrane protein"
+                     /gene="prM"
      CDS             736..960
                      /product="membrane protein"
                      /gene="MP"