Configurable celltype and nodata values

[moradology]

GeoTrellis layers can sometimes be ingested with the incorrect cell type. Nevertheless, we'd like to support using them. Similar issues can plague the use of 3rd party GeoTiffs

1. GeoTrellis layers should allow for configuration of a CellType override
2. All layers should allow manual setting of a NoData value

[moradology]

My initial guesses here seem to have been wrong and ND handling wasn't actually the fundamental cause: Int.MinValue showing up in my printed histogram seems like it was a bit of a red herring and it now appears as though MAML might be incorrectly handling the celltype in our example ned-slope layer. Further investigation is advisable