Avoid crash in file loader for all `std::exception`s
Save source points in hcloud format

* Machinary to save original points in hcloud format so that they're
  displayed when zooming in all the way.

* PageCacheReader convenience class to make it simpler to read from a
  PageCache in a sequential fashion including error tracking.

* Remove accidental unistd.h include

* New hcloud index format => increment version to 1

* HCloudView support for new index format + special cases for displaying
  points at the most zoomed in level rather than voxels.