Support for 'maxColSizeMb' autoscaling config parameter

[rrowlands]

Allows for an additional optional autoscaling configuration parameter 'maxColSizeMb', which defines the total size of the collection to be processed. This was identified as a useful parameter particularly for high resolution imagery, given that the size of individual images can be as large as 20 or 30 MB per file and subsequently take up a lot more memory on the processing node.