Avoid forcing data type for image arrays

In some places we force the dtype to be uint8 (i.e., integer on interval [0-255]). See [package.py](https://github.com/developmentseed/label-maker/blob/75edaec255cc45190e6ad690e240fb215541b4a7/label_maker/package.py#L83) for an example.

In some situations, we might want to package tifs or raw satellite imagery where the data either isn't made up of integers or doesn't fall on this [0, 255] range. For example, it might be pre-scaled from [0-1] or extend beyond the standard range of RGB vals.