Breaking partitionLociByApproximateReadDepth into multiple functions

I just took steps 1/2/3 at face value, but I'm open to this not being the ideal way to break up the function
Maybe rename this function to something like "readsPerPartition"? The "micro partitions" terminology is specific to the partitionLociByApproximateReadDepth case. Now that this function has been factored out, we shouldn't have to think about "micro partitions" here. This function just takes a partitioning and some reads and returns how many reads overlap each partition.