Separate IO logic from processing logic

[dnomadb]

nodata/nodata/blob.py

Line 58 in fa5fd93

def blob_worker(srcs, window, ij, globalArgs):

We currently have a large about of reads / writes interspersed into a function where processing paths are:

1. Decided via nested ifs/thens; and
2. Combined with working logic, eg

   nodata/nodata/blob.py

   Lines 73 to 75 in fa5fd93

   	img[-1] = (np.invert(img[-1] < globalArgs['maskThreshold']).astype(img.dtype)
   	* np.iinfo(img.dtype).max)
   	mask = img[-1]

We should try to separate these out as much as possible.