When exporting large point clouds, RAM usage can be too high and app exits

[matlabbe]

Related to this issue: #1368

Using min and max limits of Node filtering option helps to export one section of the map after the other, but it is tedious to estimate if the density of nodes is different across the map. Setting wrong limits will make the app crash if RAM is filled. We could add an option of export nodes up to a target max RAM usage, then automatically start a new cloud when limit is reached, saving the previous cloud (with some counting suffix using a base file name), clear memory and continue with next nodes.