Add information about multiple patch 'sets' to documenation

[rwood-97]

Patch sets: When running the loader.sliceAll() function multiple times (e.g. for 100 pixels and then for 150 pixels), the default option is to keep all patches. This is due to the default value of “add2child=True” that is not mentioned in the User Guide. When running loader.list_children() all patches of all sizes will be listed. This should be clarified in the documentation. - I find it very useful, by the way, that I can slice in different ways and keep all the patches so that objects cut in half by one slicing would appear intact in the other patches. This could improve the results.

Split from #118

• Add to docs explanation of the add2child function (I think this is renamed to add2parent in PR align language used to describe task names within code and documentation #95).
  This should probably go in the 'extended' docs we discussed during meeting with Jon.