Array.fill is really slow, I made an implicit way to do it faster that you get when you do import geotrellis._  

see https://github.com/geotrellis/geotrellis/blob/master/src/main/scala/geotrellis/package.scala#L39

```scala
def apply(len: Int, fillValue: Byte) = new ArgWritable(Array.ofDim[Byte](len).fill(fillValue))
```

There's 6 Array.fill usages in this PR.