Fixed colors for grade

NEST/osprey/pull/31#issuecomment-14703)

Also another good-to-have, but could-be-added-later thought. Based on some of my recent requests, since we usually use grade as `block_color` if not `NULL`, whether we could set that if the `block_color` is grade (or if `bar_color` in `counts_r`, `counts_l` is 1-5?), the default to be
```
scale_fill_manual(
name = "Grade",
values = c(1 = "darkgreen", 2 = "blue", 3 = "orange", 4 = "red", 5 = "black")
)
```

Provenance:
```
Creator: gogonzo
```