thinnedTrees() with multiple tree batches

[ms609]

in app.R, thinnedTrees() is naively defined as as.integer(seq(keptRange()[1], keptRange()[2], by = 2 ^ input$thinTrees))

This assumes that trees have been loaded from a single file. Otherwise the numbers are garbage.

It'd be nice if this didn't have to be the case. Failing that, we should disable the option if it's not relevant.