Add view position controls to API to facilitate a groundwindow/peakfinder app

[goatchurchprime]

This example works (on a phone) and uses the gyros/accelerometer to create a window through the ground. It also used to work from the GPS (before that stopped working) so you could walk around to find surface passages.
http://goatchurchprime.github.io/groundwindow/groundwindow.html#caves/expoloser.js

I'd like to have enough features in the CaveView so I can use it as a library and delete most of the rendering parts (and 3d file decoding components) of groundwindow.